Important Diseases of Crop Plants
Micro-organisms often cause diseases on plant products in the field and during storage. The most important of these are bacteria, fungi, viruses and nematodes.
Although their effects may not cause total destruction of produce, the loss in quality due to spoilage and contamination are often significant.
In this article, diseases of economic importance which affect arable and permanent crops and methods of their management are discussed.
Important Disease sof Crop Plants
1. Bacterial Diseases
Bacteria seem to be the most primitive living organisms. The bacterial cells are very small, in fact so small that in some, such as in mycoplasma, the individual cells are not seen in the light of optical microscope which can magnify an object up to 200 to 300 times.
Cellular size of bacteria (including mycoplasmas) varies with the stage of life cycle, nutritional status, mode of cell division, and many other factors.
The size is variable more in length than in width. The ordinary bacterial cell is about 1 to 5 microns long and 0.5 pm wide (1 micron = 1/1000mm). The known range of size extends from 0.1 to 0.4 microns (nm = 1/11000 microns).
The association of bacteria with animal disease anthrax had been known since 1850. The first evidence of bacteria being responsible for plant diseases was reported in 1882 when association of bacterium (now known as Erwinia amylovora) was established with the fire blight diseases of pear.
Since then the number of plant diseases identified as caused by bacteria has risen rapidly. Examples of bacteria diseases include the following.
2a. Cassava bacterial blight(CBB)
It is the most wide spread bacteria disease of cassava in Africa and ccurs in all cassava growing areas of the world. The pathogen is Xanthomona campestris pv. Manihoti. Symptoms are angular water – soaked leaf spots.
Spots enlarge and coalesce with time resulting in leaf blighting, wilting, defoliation, and die back. Severe defoliation leads to bare stem referred to as ‘candlesticks’.
Bacteria exudates on lower leaf surface, petioles and stems occur in highly susceptible varieties in periods of high humidity e.g early morning hours. It is a systemic disease characterized by vascular necrosis (usually brown in colour) of stem and roots.
CBB symptoms appear in the rainy season. It survives in the cassava canopy, vascular tissues of stems seeds and dry crop debris.
Use of sanitation measures such as destroying plant debris to ashes with fire is effective in the control of CBB. Also effective are ploughing and ridging of the plot before the next crop, avoiding unclean cutting, insect – infested cutting or mechanically damaged cutting and the use of improved resistant varieties such as those mentioned under ACMV.
Xanthomonas campestris (a bacterium) also caused leaf blight in rice especially in Asia, but was found recently in West Africa. Symptoms are chlorotic water – soaked lesions along the leaf margin. Young lesions may have bacterial exudates.
Halo blight is caused by Pseudomonas phaseolicola (a bacterium) on Phaseolus vulgaris. Symptom is small necrotic flecks surrounded by chlorotic areas on the leaf. It survives in soil, seed and diseases vines.
Control is by use of resistant varieties; so also is the use of disease – free seeds. Control can also be by applying seed – dressing with bactericides or fungicides which have bactericidal properties at 2 – 3g/kg of seed.
2. Fungi Diseases
The fungi are plants without green pigments necessary for photosynthesis. Many of them can be seen with the naked eyes.
They are either saprophytic – feeding on dead decaying organic matter (facultative parasites) or parasitic, attacking and feeding at the expense of a living host (obligate parasite).
Most plant diseases are caused by fungi. Out of 10,000 described species, more than 8000 species are of plant pathogens.
Fungal diseases such as the mildew, heat rust in cereals have been known from the Roman times. Today, fungi diseases have risen to greater economic and scientific prominence. Examples of fungi diseases include the following:
2a. Downy Mildew
This is an important fungi diseases characterized by pale chlorite patilies or streaks on leaves, and a downy white sporulation (of conidial state of the fungus) on leaf underside especially on mornings, when there is a lot of dew. There is also necrosis, retarded development and sometimes sterility.
Downy Mildew is caused by Peronosclerospora sorghi on maize and sorghum and by Sclerospora gramunicola on millet. On wheat, it is caused by Erysiphe gramunis in temperate countries.
On peas, Downy mildew is caused by Personosphore viciae, while mycelium of the fungus appears on lower leaf as in cereals. Upper leaf surface becomes yellowish, then darkens and dries up.
Control is by the use of resistant varieties and crop rotation. Infected plants should be destroyed immediately as the symptoms appear, to avoid spread. Early planting with the first rains is recommended in maize (susceptible in the first 4 weeks).
When the rains increase and humidity increases, the fungal spores multiply and spread fast. The chemical “Apron – plus” may be used as seed treatment.
Resistant maize seeds such as DMR and DMR – ESR (W&Y) may be obtained from NSS. Commercial seed companies and state ADP’s in Nigeria. Crop sanitation and rotation is recommended in sorghum.
Rusts are caused by the basidiomyceters of the fungal order Uredinales. Puccinia sacchari and P. kuehnii on sugar cane, Puccinia polysora on maize, Puccinia graminis on wheat (very destructive) and Puccinia on millet.
Rusts are characterized by small rusty brown pustules on leaf surface with yellow uredospores within. Leaves become chlorotic and dry up when severe. On millet, the pustules are elongated and orange in colour.
Spread of rusts is mainly by wind – borne spores. They are also spread by rain and animals. In the infected plant, they reduce the rate of photosynthesis and increase respiration.
Photosynthates produced are diverted to infected tissue, thus yield and quality of grain produced are reduced. Losses caused by rust may amount to 10% of world grain crops.
It is one of the most destructive plant diseases affecting mostly leaves and stems, sometimes flowers and fruits. They occur mostly on cereal crops but also affect tree crops and ornamentals.
The method of control is by the use of resistant varieties.
Smuts like rusts are caused by bacidiomycetes of the fungal order Utillaginales. They are characterized by dark powdery masses of spores which look like soot/smut.
They are as economically important as rusts and also attack mainly cereals, but also infect onions and ornamentals, causing significant reduction in yield of kernels.
As they develop, the plant tissue is destroyed and replaced by a mass of block spores covered by thin membranes referred to as sori (singulars; sorus). Example – In head smut caused by Spaceiotheca reliana in maize, sori take over the inflorescence. Also, in the covered kernel smut of sorghum caused by Spacelotheca sorgi, grains are replaced by sori.
Seeds should be treated with fungicides such as thiram and carboxin to prevent seedling infection. In the latter, the seed – borne fungus develops in the growing seedling, infecting the inflorescence as it is formed.
Sometimes, infected tissues are stimulated to divide and enlarge into galls (hyperplasis), and then they are destroyed and replaced by a mass of black spores. Examples: common smut in maize caused by utilago maydis, where gall – like growths occur in the aerial part of the plant.
Galls are more commonly formed in the cobs. In some cases, smuts may cause local infection only on leaves and stem. Spores of smuts are usually spread by the wind when the covering enclosing them is broken during harvest or threshing.
Survival is usually by spore known as telisporea in solid plant debris or on contaminated seed. Some may survive as mycelium in infected kernels or infected plant.
Control is achieved by planting of resistant varieties. However, as meiosis (genetic recombination) takes place in every generation of smut fungi, new races of the pathogen are constantly being produced, therefore breeding for resistant varieties to keep pace with this must be continuous. The latter is not very easy.
2d. Blast disease of rice
It is caused by a fungus called pyricularia oryza. Dryness predisposes plants to infection. Thus it is the most serious disease of upland rice. At the early storage of the crop, it causes seedling blight, later in the development of the crop, it causes leaf blight.
At the reproductive stage (formation of heads) it causes neck rot, thus yield is lost. Symptoms are necrotic spots on leaves which develop brown margins with grey centres. Spots latter merge and leaves becomes brown and shriveled.
Humidity of over 90%, temperature of 24 – 270C and excessive nitrogenous fertilizer favours the disease. It is seed and airborne. Best control is by the use of resistant varieties. Fungicides such as isopothiolane and antibiotics such as blasticin are economical only on seeds.
2e. Stalk and earrot
It is caused by Diplodia maydis (fungus) which also causes seedling blight in maize. Symptoms of stalk rot are yellowish brown on lower nodes. The stalks become brittle and break. Dark conidia are usually present under the epidermis.
Other fungi causing stalk and ear rots include the following: Fusarium graminearum (Gibberrella zea – Ascomycete) and Fusarium moniliforme (G. fujikuroi – ascomycete) in sorghum. Infected tissue is reddish or brown coloured in sorghum. A characteristic pin mould is formed on infected maize grains especially in wet weather.
Seed – borne infection can be controlled with seed dressing fungicides. Use of resistant varieties (e.g flint) rather than dent varieties are ideal; so also crop rotation.
2f. Ergot(Sugary or honey dew disease)
Caused by Clavicepts fusiformis – Ascomycete (Spacelia sorgi = imperfect state) in millet and sorghum. Young flowers infected with the conidia of the fungus usually produce a creamy sweet tasting liquid in the ovary. The liquid attracts sooty mould and darkens up.
The disease derives its name from the sclerotia or ergot (about 5mm long) produced when the flowers mature. The ergots stick out among the grains and are normally harvested along with the grain. They are known to be poisonous when eaten (along with the grains) by man or livestock.
Control is by early planting (in late planted crop, stress sets in and plant flowers early in order to complete life cycle) and ploughing to bury sclerotia/ergots and reduce incidence. Research on use of resistant varieties is still at the development stage.
2g. Leaf Spots
Brown spot of rice is caused by Cochliobolus miyabeanus (syn. Helminthosprium orzea. Leaf spots are also caused by Cercospora orryzae on rice (narrow leaf spot), Cercospora sorghi on sorghum (grey leaf spot) and Curvularia penniseti on millet. Disease is seed – borne and control is by burning infected crop residue; Also seed – dress with fungicides.
It is caused by Celletotrichus spp. in different crops. For example, the disease is caused by celletotrichum gramminicolum on sorghum. Symptoms are red/purple spots on leaves, and also red rot on stem leading to stem lodging.
Leaves dry up and grains are not well developed. It also causes leaf spots on maize, as well as reddish brown sunken spots on leaves and stem of sorghum. Control is by the use of resistant varieties.
In Vigna spp., it is caused by the fungus collectrichum hindermuthianum where the symptoms are lesions red in the centre, with acervali and brown to purple margin. It usually infects the aerial parts of the plant.
It is responsible for the well – known blotch disease of cowpea which causes the purplish brown discoloration of petioles, leaf veins, stems, peduncles and especially pods. Infected pods become distorted.
Pod loss makes it an important disease of cowpea. C. capsici and C. truncatum also affect cowpea. On seeds, the lesions are brown in colour.
Control is by the use of crop rotation and resistant variety as well as clean certified seeds. Seed dress with fungicides like benomyl, captan or agrosan. Also 2 or 3 applications of benomyl at 15g/10 litres of water are recommended from flowering. Alternatively, dithiocarbarmae at 1kg/ha in 500 to 1000 litres of water can be used.
The disease is also caused by the fungus Collectotrichum gloeosporiodes(Glomerella cingulata) on yam and G. manitholits on cassava. In this case, dark brown to black lesions appear on leaves, petioles and stems of yam.
If humidity is high and temperature is between 28 and 300C, the lesions enlarge and coalesce, giving rise to extension necrosis of stem leaves and stem die – black. Symptom is depressed brown oval lesions especially on cassava stem. Later lesions become raised, fibrous and deep cankers. It survives on crop debris, soil and tubers.
Use of resistant varieties is best control. Mixed cropping and staking reduces disease incidence. The uses of crop rotation disease – free cuttings are also effective in control.
It is caused by a special form of Fusarium omysporum on cowpea and pigeon pea. It is reported on cowpea in Asia, Australia, North and South America but in Africa, it has been reported only in Nigeria and Uganda. It is soil and seed – borne.
It is characterized by gradual or sudden wilt, yellowing and drying of foliage, then plant collapse and die. In young plant, the wilt is rapid, leading to death. Older plants are stunted with symptoms mentioned above.
The disease is systemic, evidenced by necrosis of the vascular tissue in cowpea or blacking of stem base and internal root rot in pigeon pear. Control by using clean seeds, resistant varieties and crop rotation are effective against the disease.
Helminuthosporium turcicum (a fungus) causes leaf blight in maize and sorghum. It is widespread and destructive. Symptoms include linear elliptic brown water – soaked lesions on leaves and leaf shealthies. Leaves are destroyed when severe. Main source of innoculum is crop residue. Control is by the use of resistant varieties.
Leaf blight caused by Helminthosphorium maydis also occurs in maize. Symptoms are reddish brown lesions, smaller than those of H. turcicum between the veins. Control is by seed dressing with fungicide.
Rhizoctonia solani causes leaf blight in millet and sorghum and sheath in rice. Sheath blight which is important in paddy rice is characterized by grey elliptic lesions on the sheaths at water level. Lesions develop into irregular elongated botches with pale centre.
Schlerotia are formed under the sheaths. Infection may extend to leaves or stem. Survival of the pathogens is as dormant mycelium on the water. Best form of control is host plant resistance. Cultural control method includes adequate spacing, destruction of innoculum sources and avoidance of excessive nitrogenous fertilizer.
Blight is caused by Coricium solani (Rhizoctonia solani, Thanatephorus cucumeris) on cowpea and referred to as web blight. It is characterized by small red – brown spots on leaves which become surrounded by irregular water – soaked areas.
Lesions coalesce rapidly in humid weather and mycelium is present on the undersurface of leaves and young stems. It is an economically important disease in West Africa where crop loss may be total in periods of heavy rains. This pathogen survives in soil and crop debris.
Control is by use of clean seed. Also avoid dense sowing in periods of high rainfall.
Blight caused by Sclerotium rolfii is characterized by brown sunken lesion on leaves and stems and white mycelium near the roots; Sclentia varieties are also present in infected areas. Infected plants wilt and die. Control by use of resistant varieties and crop rotation with cereals are affective.
Taro leaf blight on cocoyam is caused by Photophthora colocasiae. Symptoms are circular purple – brown lesions with yellow halos/surroundings on leaves.
Later, lesions coalesce to give extended leaf blights leading to leaf death and poor corn development. Sometimes there is plant collapse. The fungus survives in infected materials.
Phytophthora infestans causes late blight of potato. The water – soaked spots on the leaves coalesce resulting in large blighted areas.
Stems and tubers also become infected. In favourable (cool) weather, infected caves shrivel and die, while tuber infection advances in storage. Phytophoria infestans also cause leaf blight, stem blight and die back in tomato and Amaranthus.
Control is by gathering and burning crop refuse and alternate hosts. Well – timed fungicidal spray of Dithane M45 at 20g/10 litres of water weekly, or Difolatan at 18ml/10 litres is also effective. Also use resistant varieties and rogue blighted plants.
Rust disease is caused by Uromyces spp., U. appendiculatus on phaseolus spp. and Vigna spp., and U. cajani on cajanus cajan. Symptoms are red – brown pustules with yellow chlorotic surrounding halo on leaves which later become dark brown or black.
When severe, there is defoliation. Spores are wind – borne and survive as teliospores on volunteer/alternative legume crops and crops debris. Control is by the use of resistant varieties.
Read Also : General Field Activities and Operations in Crop Production
Coffee rust disease
This is caused by the fungus hemilieiu vaststrix and it an important disease the world over. It is characterized by orange – yellow powdery spots on the leaves. Spots enlarge, coalesce and turn brown, then dry and fall prematurely. Yield is reduced. Infected trees may die later.
Bordeaux mixture at 2 to 3 week interval should be applied during the rainy season as control measure.
Leaf and PodSpot
It is caused by Ascochyta pisi on peas. Small purple spot appear which enlarge and turn brown or black. Leaves shrivel and dry. The disease is both soil and seed – borne. Control is by seed dressing with argrosan at 2 – 3g/kg of seed, use crop rotation.
Angular leaf spot is caused by Phaeisariosis griseola on Phaseolus spp and Vinra spp. Small grey angular spots limited by vein on leaves are formed, which later become dark – ash colour. Lesions appear on stems and pods. The fungus survives in seed and crop debris. Control is by seed dressing with argosan at 2 – 3 g/kg of seed and use of crop rotation.
Brown leaf spot of cassava is caused by Cercosphora henningsii. Spots are round and brown on leaves. They then expand, coalesce and become angular due to delimitation by small veins.
Infected leaves become chlorotic/turn yellow, dry and drop. Survival is in seeds or as black stomata (compact mycelial structure) in infected leaves. Leaf spots may lead to about 20% yield loss in areas where rainfall exceeds 120mm.
Leaf spot of groundnut. It is caused by Cercospora personatum and C. aracnidicola.
Losses caused by these fungi are equal to half of that caused by other diseases on groundnut. Symptoms are circular spots with yellow hale, later changing to reddish brown or black. Symptoms usually appear when crop is 4 – 8 weeks old and persist till harvest time.
There could be complete defoliation with heavy spotting. Conidia are wind – blown and survive in crop debris. Use of resistant varieties is the best control. Crop rotation and seed dressing with fungicides are also effective.
Plantain and Banana (Musa paradisiacal and M. sapientium) Sigatoka or Banana leaf spot disease. This is caused by the fungus Mycosphaerella musicola (an ascomycere). It is one of the most destructive diseases of Banana and plantain and sistribution is worldwide.
Symptoms are light yellow spots parallel to the side viens ofn the leaf. The spots enlarge and become dark brown. The centres of the spot die becoming grey in colour. A yellow chlorotic area surrounds each spot and this is in turn surrounded by a brown ring.
Steaks occur on the petioles also. When severe, the spots coalesce and large areas of leaf die. Number, size and quality of fruit in terms of taste are reduced. Also, there is premature ripening. Where diseases reduces the number of leaves on plant to less than 12, fruits may not fill out, are undersize or may drop.
The best control is with application of fungicides such as benormyl and manczed, suspended in an oil water emulsion, applied as frequently as forthigntly or monthly. Plant spacing to reduce disease incidence is also effective.
A cercespora state of the fungus exists, which produced spores at the brown spot stage. These are dispersed by water splash. Perithecia containing asci are produced in the grey centres of the lesions. From these ascospores are forcibly released in wet weather and carried in the air to long distances.
Tobacco (Nicotiana tabacum) Cercospora leaf spot disease spots are small, brown and roughly circular with reddish purple borders. Centre of spots later become grayish, thin and brittle often, falling out. Causal fungus is Cercospora nicotianae.
As a control measure, apply thiabendazole fungicide as a spray at 30 to 120g. a.i. per 100 litres of water weekly or biweekly at first sign of disease.
Rots.Examples include the following:
Black rot of potato is caused by ceratocystis fimbriata (Ascomycte). Undwerground plant tissues such as rootss and tubers usally affected. Symptoms are sunken black leions. The spread of infection to the stem results in the characteristics “blank shrank” or a black necrosis of the stem.
The fungus survives in the soil and infected tubers. Control is by the use of crop rotation and use of disease – free material. Corm rot in cocoyam is caused by Phythium spp and Phythophora spp.
Cassava soft root rot is caused byPhytophora spp; Phytium spp and Fusarium spp.
Symptoms are brown necrotic lesions on roots tube. As the latter decays, plant wilts and dies. A characteristic foul odour is emitted from infected roots. Survival is in infected roots and soil. Yield loss may be up to 80% in high rainfall areas.
Mature tubers are predisposed to infection when harbvest is delayed. Control is effective by crop rottationm with non – host crops. Also, avoid continous monocropping of cassava in high rainfall areas.
Sclerotum rot is the most common cassava tuber rot disease but yield loss has not been quantified.
a) Citrus foot rot (gummosis) is caused by Phylophtora spp. The tree trunk is infected at or below ground level resulting in death of the cortex. Infection is accompanied by gum exudation. Control is by planting of resistant varieties.
Stem rot of tobacco caused by Scolerotinia sclerotiorum. Fungus is soil borne and may affect any part of the plant at seedling stage. In the field, fungus causes stem rot resulting in the stem tilting to the ground and thus infection of the leaves touching the soil.
Control is by applying thiabendazole 60 WP, at 400 – 900 a.i per hectare, 30 – 50 days after transplanting, then weekly according to disease severity. The thiabendazole may also be used at 60g i.a dry or as a water suspension per 100kg completely dry tobacco to control mould by Aspergillus and pennicilium in storage.
For young plants, (3 to 4 leaves) in seed beds, thiabendazole 60 WP may be applied 10g per 50m2 at 7 – 10 day intervals for about 30 applications. The leaf spots and field spot and disease of tobacco are world – wide.
b) Cocoa (Theobromae cacao) black pod diseases. Black pod disease is caused by Phytophthora palmivora and is the most widely distributed disease of cocoa. It is characterized by a dark brown or black rot of the pod. Internally, the beans are partially or wholly destroyed.
The fungus also attacks seedling, flowers, cushions and the stem cankers. It is spread by wind, insects and splashes of rain from diseased to healthy pods.
It survives in the soil and diseased tissues.
Control is by spraying Bordeaux mixture at monthly intervals before pods reach maturity. Perenox (Cuprous oxide) may also be used. Infected pods should be removed as soon as they are noticed in the field.
Damping – off of seedlings may be caused by fungal genera such as Pythium, Rhizoctonia, Sclerotium, Phytophthora and Fusarium.
Control: destroy crop debris and alternative hosts. Use Dithane M45 or use Difolatan as seed – dress or Benormyl at 4.5gm/11 of water.
The word “virus” comes from the Latin and means a poison. More than 300 plant viruses are known. Viruses are microscopic and visible only with an electron microscope. They are infectious, obligate parasites that need living cells in which to replicate.
Viruses are transmitted from diseased to healthy plants through wounds caused or animals, including insects, other arthropods, and nematodes. Viruses can also be transmitted by fungi, parasitic seed plants, contaminated seeds and pollen contact or friction between plants and through injuries caused by people and machine.
The morphology of the virus particles is varied. They may be polyhedral, bullet rod – shaped. Rod – shaped various vary in length from 100 – 1200nm, and polyhedral vary in diameter from 18 to 70 nm.
Plant viruses are identified by their host range, mode of transmission, and physical and chemical properties. They are named in the vernacular, usually with a compound of the names of the first host on which the virus was observed and the type of symptom noted e.g. maize dwarf mosaic virus. Not all viral names however are associated with the name of the causal virus.
The effect of viruses is systemic and leads to the occurrence of such symptoms as yellowing, mottling, leaf curl, excessive branching gall formation or stunting in infected crops. Severe infection leads to death or reduction in yield. Examples of viral diseases include the following:
Cassava mosaic disease
Cassava mosaic disease is caused by African cassava mosaic virus (ACMV). It is the most wide spread disease of cassava in tropical Africa and India. Vector is the Bemisia tabasi (white fly).
Symptoms are mosaic pattern on leaves. Also there is pucketing, distortion and reduction on leaves. When severe, the plant is stunted and leaves reduced to veins.
ACMV may move down to the shoot but is not always fully systemic. Yield loss of between 20 to 60% is usual on crops. Use of resistant varieties is the best.
Examples are TMS 3057, 30555 and 30573, Anti – ota. Ofege (MS – 6), Idiogbayepe (MS – 20). Quarantine, sanitation and manipulation of planting date to avoid period of high spread are also recommended.
Streak disease of maize
It is caused by maize streak virus and characterized by narrow chlorotic streaks along the leaf veins. Usually the schlorotic streaks are distributed over all the leaves and this makes it distinguishable. Transmission is by leafhoppers of the cicadulina sp.
Control is by the use of resistant varieties such as TZSR, TZERS, and DMR – ESR. Sugarcane mosaic virus disease also affects maize.
Tristeza virus disease (or quick decline disease) cause stunting, stem pitting and death of the tree. Control by using resistant or tolerant nursery stocks, or tolerant scion – stock combination.
It is recommended for economic reasons that susceptible trees be removed when infected and replaced by resistant combinations.
Swollen shoot disease of cocoa – caused by cocoa swollen shoot virus. It has produced great losses in West Africa. There is abnormal growth of the vascular tissues so the shoot becomes swollen.
In addition, there is mottling, vein flecking and leaf mosaic. Also, there is leaf fall, die – black and subsequent death of the tree. Infected trees lose vigour and there is loss in yield.
Removal and burning of infected trees has been the means of control so far, but it is expensive as it represents depletion of resources. Chemical control of vectors give only partial control. The mealy bugs Pseudococcus njalensis and Planococcus citri transmit the virus.
Control of plant viral diseases:
There are various methods of approaching the control of the spread of plant viruses; they are not applicable in the same way to the various diseases. These methods can be classified under the following six headings:
1). Elimination of the sources of virus infection. The following are important for note:
Weeds, woody shrubs and trees are frequently host to viruses, many of which are transmitted through the seeds of such plants.
Viruses which have a fairly wide host range can be brought to one crop from another cultivated crop.
For example, clover is the host of several viruses which affects peas and beans; moreover, the chief aphid vector over – winters on clover. It is therefore advised to grow perennial leguminous crop in close proximity to susceptible annual crops.
Remnants of the previous year’s crop are frequently important sources of virus.
With vegetative propagated crops, such as cassava, it is extremely important to start with a virus free crop.
Any obviously virus – diseased plants should be rogued out as early as possible while the plants are still small.
Avoiding the vectors
This can be achieved in various ways, by isolation, by breaking the cycle of the vector, virus and host plant, and by artificial barriers to exclude the vector.
For example, it is possible to raise virus – free seed potatoes in areas where the temperature is too high for the aphid vector by early sowing e.g. early sowing of maize avoids high infestation of Cicadulina, the vector of the maize streak virus. Barriers can be insect – proof cages or barrier crop like maize or sun flower.
Direct attack on the vector
Virus diseases transmitted by aphids and leafhoppers can be controlled by applying insecticides, especially persistent and systemic insecticides. It is not sufficient to kill the insects which have been bred in a crop; they must be killed as they enter a crop.
However, insecticides can neither present the introduction of a virus nor its spread within the crop. This is because when an infective insect lands on a sprayed, it can infect plants before it dies. Also, non – infective insects arriving upon a sprayed crop could acquire and transmit certain viruses before dying.
Breeding resistant varieties of crop
This is one of the most promising methods of control. Some varieties of cassava are resistant to the cassava mosaic virus while several varieties of maize are resistant to the maize streak virus. These varieties have reduced the importance of the two diseases:
Cure of virus– infected plants
Viruses can be eliminated from infected plants by inactivating the viruses by heat or with chemicals.
For example, Ratoon stunt, a serious virus diseases or sugarcane, is controlled by exposing the sets for 2 hours in hot water at 500C.
Special method of propagation
By taking advantage of the rate, or lack, of movement of a virus in plant, it is sometime possible to propagate from tissues which are temporary free of invading virus.
This is a useful technique in cases of valuable plants or where it is desired to build up virus free done of a particular variety since some viruses fail to invade the growing point, the apical meristem may be cut off and grown in a tissue culture.
Nematode as pest has been discussed in module 1, unit 3. Typical symptoms of nematode damage is irregular areas of varying size in which the pants have been unhealthy appearance.
Heavily infected plants are smaller than normal plants, are usually chlorotic, and have a tendency to wilt because of reduce or unhealthy root system.
Below ground symptoms vary, depending on the specific nematode attacking the roots. The following diseases symptoms are common;
Root knots orgalls: Root tissue close to a nematode’s head often becomes enlarged bulbous and distorted.
Lesions develop when migratory endo – parasitic nematode enter and move within the parenchyma cells of the hosts roots. As the nematodes feed, cavities develop, which may result in falling off of cortical tissue.
A small root often become girdled by such injuries, so that root pruning occurs. Death necrosis of root tissue is often attributed to micro – organisms that enter roots via wounds caused by nematodes.
Abnormal or reduce root development
Ectoparasitic nematodes normally feed on root tissue nears the meristematic and cell elongation regions. Damage to or death of root cells is primarily caused by the repeated probing of the stylet into the tissue. The plant then develops short thick tissues.
Nematode diseases of important crops include the following:
1. Root–knot and cyst nematodes diseases of legumes
They casue the greatest yield losses on most legumes. On cowpea, root – knot nematodes casue root galling and floss of yield of cowpea.
NM. Incognita, M. aouta and M, javanica are the major root – knot species found on cowpeas in most areas. M. incognita also causes serious galling of winged bean roots and tubers as well as pigeon pea.
The above ground symptoms resulting from damaged roots are yellowing, stunted growth, wilting, nutrient deficiency symptoms, poor yield and early senescence.
These symptoms often appear in early defined patches, in the field and are more obvious in adverse growing condition, e.g. drought, and in sandy soils. Yield losses in the tropics have been estimated at 8 – 35% for pigeon pea and 10 – 43% for cowpea.
Cyst nematodes Helerodera cajani are also common to pigeon pea, cowpea and other legumes. On several infected plants, the mature, swollen white females are clearly visible covering the surface of the roots.
Brown cyst containing valuable eggs can survive in the soil for many years. Field symptom on susceptible legume host attacked by Heterodera appear as yellowing.
2. Root–knot and cystnematodes diseases of cereals
The disease manifest as stunted roots and poor developmenmt in maize and other cereal crops – Trichodorus, Belonolanimus, Xiphinema. The cereal cyst nematode Heterodera avenae causes deformed root and poor development in wheat.
Nematode diseases of roots and tubers:
Scutellonama bradys. Internal symptoms are small yellowish lesions under the tuber skin which later turn dark – brown to black. Then lesions coalesce forming a continuous dark, dry – rot layer which may girdle the whole tuber.
External symptoms are slight deep cracks in the tuber skin or flaking – off of epidermal layer exposing the internal dry rot when infections is severe.
Tubers are also malformed. Storage losses as high as 80% have been recorded. It reduces edible portion of tuber and their market values.
Lesion nematode in Yam – Pratylenchus spp. Infected tuber shows severe necrosis resulting in a dry rot and deep cracks in the outer tissues, skin splitting and corky patches.
Edible portion and market value of infected tubers are reduced.
Root knot nematodes infecting yams – Meloidogyne spp. Symptoms are slight to severe galling of yam roots which later results in irregular knobby tuber.
Infected seedlings are stunted with galled roots and foliar necrosis. Meloidogyne reduces yam growth and caused yield losses in then filed. It lowers quality and market values of infected tubers.
Control methods of root knot and cystnematode diseases:
Chemical method: Nematicides are the most effective means of reducing yield losses by root – knot nematodes, but always the most economic. The following have been recommended for the control of nematodes on cowpea.
Methyl bromide injected by chisel application at 20cm depth at 224kg/ha under polythene covers.
Ethylene dibromide at 17kg/ha as a pre – plant fit.
DBPC injected 2 weeks before planting at 5 – 7.5 ha a.i/ha.
Aldicarb as a 30cm band pre – planting and side band post – emergence application at 1kg a.i/ha.
Rotation of crops is recommended but selection of poor or non-host crops should only be done after careful screening. Susceptible legume varieties should not be grown successively on the same land, and an ideally an interval of 2 – 3 years is recommended.
Planting of legumes and other susceptible crops (e.g. solanaceous, cucurbitaceous and other vegetables). Crops which have been shown to reduce root – knot population include cereal (sorghum, millet, maize, wheat, rice) cruciferous (cabbage, cauliflower), onion, garlic, groundnut.
Cotton and rosette (Hibiscus sabdariffa) the use of crop will depend on the root – knot species or race present. For example, groundnut and cotton are susceptible hosts of M. arenaria and M. acuta, respectively. Crotalaria has been recommended in Brazil in rotation with cowpea to control M. Hapla.
Some intercropping plant species are known to produce root exudates which are toxic to root – knot nematodes, e.g. tangerine marigold (Tagetes patula) and sesame (Sesamum indicum) and these have been used with limited success by intercropping with root – knot susceptible crops.
A fallow period before planting a susceptible crop will greatly reduce root – knot, but weeds can also be hosts for the nematodes and a bare fallow is recommended turning the soil during the fallow will give additional kill of the nematodes.
The use of nematode – resistant crop varieties is recommended as an economic means of managing root – knot nematodes. Many cowpea varieties and breeding line have been found resistant to meloidogyne spp.
Control of nematode diseases in root and tubers is by planting nematode – free tubers in soils where nematode population has been eliminated or is in minimal, as infection starts from field.
The latter are soils which are left fallow for appropriate periods. Avoid intercropping or rotation with crops which are alternate hosts such as vegetables and legumes.
Diseases of stored products
Fungi and bacteria may infect fruits, vegetables, cereals and legumes in the field. In storage, fruits and vegetables deteriorate further as a result of the action of these pathogens or others. Penetration of this pathogen is enhanced where cuts and bruises exist on the commodities.
In seeds and legumes, however, their action is inhibited mainly because of the drying of these products to low moisture content for storage purposes.
Diseases of fruits and vegetables in storage are mainly caused by fungi belonging to the gebnera Fusarium, Penicillin, Alternaria, Geotrichum and bacteria of the genera Erwinnia and Pseudomonas.
Stored seeds and legumes are mainly infected by fungi in the genera Aspergillus, Penicvillin spp. May also casue disease in the fruits (orange, mango etc) and vegetable (tomato, pepper, onion etc.) and tubers include Curvularia, Aspergillus, Rhizopos, Botryodiplodia, Colletotrichum, Cladosporium, Staphylococcus and Lactobacillius.
Storage disease reduces both quality and quantity of produce which in turn lower their nutritional and market values.
Effects on the quality of stored produce include;
(i) Discolorations, shriveling and rotting,
(ii) Reduction of germinability,
(iii) Selection of toxin which make produce unfit for consumption by man or livestock.
Note that heating is a sign of microbial activity in stored produce. This results from the growth and respiration of fungi. Moisture is also produced during respiration which enhances bacterial activity.
Control of diseases of stored products:
Ensure stored produce are free of diseases, insects and pests provides entry points for microbial pathogens which cause decay. Such pests may be controlled by use of chemicals.
Harvesting and other handling practices should be done with care to minimize bruises, cuts etc. fruits and vegetables should be harvested in cool, dry weather and should not be squashed or wounded.
Seeds and grains should be dried to a moisture content level below the minimum required for growth of common storage fungi.
Proper ventilation should be maintained in stores/storage structures to prevent temperature build – up and subsequent condensation of water on produce. A high level of hygiene is also important.
Storage temperature of seeds and grain should be low i.e. 120C to 150C as grains respire slowly at low temperature. Also, growth of storage fungi is slow at low temperatures, while insects and mites activity is reduced.
Storage temperatures for fruits and vegetables should also be low. However, temperature should not be low as to cause chilling injury for stored products.
Curing at 280C to 360C for about 2 weeks may be necessary for some produce e.g. onion and sweet potato, for healing of wound by wound periderm formation and suberization as well as reduction of surface moisture. (Farmer normally field air – dry onion for 7 to 10 days after harvest).
In summary, diseases of plant products are often caused by micro- organisms, particularly bacteria, fungi, viruses and nematodes.
Their effects may cause loss in quality or even total destruction of produce. Management strategies should therefore be in place to reduce their effects to the barest minimum.
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