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Wilt: The most common and destructive disease of crops 
By M. Mithal Jiskani
Assistant Professor (Plant Pathology)
Sindh Agriculture University Tandojam


Wilt is the loss of rigidity and dropping of plant parts (partially or wholly). It is one of the most common and destructive disease of many field, vegetable and fruit crops, also attack on many species of wild plants and trees. In Pakistan, wilt is reported on 28 different crops, including Cotton, Rape seed and Mustard, Groundnut, Linseed, Sunflower, Safflower, Gram, Mung bean, Moth, Mash, Lentil, Pea, Guar, Potato, Chilies, Tomato, Cucurbits (vine crops e.g. melons, cucumber etc.), Citrus (orange, lemon etc.), Banana and Guava (Table 1). Now a day, the symptoms similar to wilt disease are also occurring in wheat, maize, sugarcane, sesame, castor, soybean, pigeon pea, mango and papaya.

The wilt appears most commonly every year in the fields if once occurred and crops are sown repeatedly without rotation, and hence it causes considerable losses in every cropping season. The most of the growers are near about unaware to the problem, therefore the symptoms, perpetuation, preventive and curative control measures are given for their guidance as well as just reminder to the researchers and extension workers, with a hope that all the concerned will think about the issue and try to find out the most easy and economical ways to combat wilt diseases. It is also expected from the breeders that they will work more on only the safe solution of problem, i.e. developing and introducing resistant varieties.

The wilt diseases may be caused by fungi, viruses, nematodes, parasitic flowering plants and insects. These are usually confused with root and crown rots, stem cankers, insect injuries, drought or excess water, soil compaction, and other non-infectious problems. The wilts caused by other than fungi have no economic importance in Pakistan, but the bacterial wilt, caused by Ralstonia solanacearum is now become important one after fungal wilts. According to Ahmed and Burney (2nd International Bacterial Wilt Symposium, 22-27 June, 1997), bacterial wilt was first reported in 1968 in the potato growing areas of Punjab, Pakistan. They stated that, the prevalence and incidence of this disease on other crops in Pakistan were lacking, but recent surveys of potato, tomato and chili growing areas reveal that the pathogen is present in all the four provinces of Pakistan. They also reported disease incidence in tomatoes and chilies, and isolated and identified the bacterial wilt pathogen from diseased peanut, beans and sunflower plants. However, the extent of damage due to the bacterial wilt on these hosts has not been documented.

The most common and destructive wilt diseases are caused by several species of Fusarium and Verticillium fungi, their common names are referred to causal fungus such as Fusarium or Verticillium wilt. Both the fungi differ from one another, but cause very similar symptoms. The only reliable method for separating and identifying these diseases is taxonomy of causal fungi, both can be readily isolated from the diseased plant portions and cultured. Both the fungal wilts thrive with high nitrogen fertilizer, excessive soil moisture, thin stands, and deep cultivation during the growing season. Both the fungi survive for long periods (for at least a decade) in soil on organic matter and even on non-host plant. However, some of the differential general characteristics between both the wilt fungi may also help in identification and control (Table 2).
 

Table 2: Differential general characteristics between Fusarium and Verticillium spp.

Fusarium wilt

Verticillium wilt

grows best in acid soils

thrives in alkaline soil

more prevalent in sandy soil

more prevalent in heavy soils

causes more injury when root-knot, reniform or sting nematodes injure the roots

does not require injury for infection

can be transmitted internally in seed

can not be transmitted internally in seed

prefers hotter soil than Verticillium

prefers cooler soil than Fusarium

 

The Fusarium spp. are predominantly common soil fungi, found in almost all parts of the world as a not-detrimental colonizer of root surfaces or a weak invader of the root cortex of many plants. There are over 80 known strains only of F. oxysporum, which show specific pathogenicity to particular crops, causing the vascular wilt diseases. Mirza and Qureshi (1978) found that most of root infecting fungi including Fusarium spp. are known to attack many cultivated plants and parasitized 36 hosts in Pakistan. The cotton husks, used as cattle feed are identified as a potential source for spread of wilt causing fungus (F. oxysporum f. sp. vasinfectum).

In addition, both the pathogens (Fusarium and Verticillium spp.) are seed-borne, may carry over for long-distance, spread from year to year by sowing infected seed; also move from field to field, farm to farm and region to region, whenever very small quantities of infested soil is transported through boots, farm equipments, vehicles, in water (irrigation and overland flows), in flood irrigation and also in infected plant material, including seed etc. The diseased plant debris lying in the field or in the threshing area also helps both the fungi to remain alive. In case of banana, Fusarium wilt or Panama disease is considered the world’s worst disease and the greatest danger of its spread comes from moving infected suckers (planting material, look like healthy on the outside).

The wilts start appearance at seedling in case of early attack and/or may also occur at any plant growth stage. Symptoms include seed rot, seedling death and wilting, and death of adult plants. Initially, vein clearing and leaves show a distinct loss of turgor and yellowing occurs starting at the leaf margin between the main veins, sometimes referred as leaf epinasty, followed by stunting. The stunted plants may fruit early. The yellowing progresses inward and is followed by necrosis hence defoliation occurs starting at the bottom of the plant, and working towards the top, results progressive wilting of leaves and stem, defoliation and finally death of the plant (partially or wholly). The diseased plants are pulled out easily, due to the loss of rigidity. Brown, red or yellow rings of discolored tissue observed in case of cutting the stem in crossways, mostly close to the base. Fusarium wilt is a solid dark brown ring around the outer portion of the stem, in cross section in the area of the vascular bundles, and that the infected areas observed irregular in shape and size. The wilt diseases mostly spread in patches, but entire field may also be affected in severe conditions. Some formae speciales are not primarily vascular pathogens but Fusarium spp. may cause foot and root rot or bulb rot in cotton.

Preventive and curative control measures
Prevention is better than cure, because control of wilt diseases is very difficult, chemical control is very costly and even is impossible to recover heavily infected, shriveled and damaged plant. However, cultivation of resistant variety is only the most easy, economical and safe method. Growers are advised to plant improved immune or resistant varieties, because these are equal to the susceptible varieties in yield, adaptation and other cultural characteristics and field practices; since new races of the fungi may develop and attack on these. However, it is necessary that growers must keep in touch with the latest information on resistant varieties and control measures. The following preventive measures also help to minimize risk of damage.

• Avoid cultivation of susceptible varieties/cultivars in the diseased area at least for three years.
• Clean cultivation through deep and repeatedly tillage operations help to destroy infected plant parts from field.
• Good drainage improves soil conditions which help to minimize the disease incidence.
• Use of disease free seed or seed treatment with suitable seed dressing fungicides before sowing is also recommended.
• In some cases, late and deep sowing reduces disease incidence.
• Mixed cropping of with non host crops help to check the disease.
• Uprooting and burning of diseased plants minimize further spread of the problem.
• Avoid storage of plant waste material and even threshing in the field, otherwise sweeping the threshing floor and burning or burying all plant debris must be ensured.
 

Table 1: Crops affected by wilt causing fungi in Pakistan

Crops

Disease

Fungus involved dominantly

Cotton

Wilt

Fusarium oxysporum f. sp. vasinfectum and Verticillium spp.

Seed, seedling, foot or root rot & boll rot

Fusarium spp.

Wheat

Foot rot, head blight & black point complex

Fusarium spp.

Seed infection

F. equiseti and F. moniliforme

Rice

Associated with seed

F. equiseti, F. moniliforme, F. semitectum and F. graminearum

Maize, Barley & Sorghum

Stalk rot & seed infection

F. moniliforme and F. semitectum

Sorghum

Grain mold

Fusarium spp.

Sugarcane

Pokkah boeng

F. moniliforme

Rape seed & Mustard

Wilt and root rot

Fusarium spp.

Groundnut

Wilt and root rot

F. coerulium & its other species

Sesamum

Wilt, stem & root rot

Fusarium spp.

Linseed

Wilt, root rot & seed infection

Fusarium lini, F. solani and F. moniliforme

Sunflower

Wilt and root rot

Verticillium dahlae

Safflower

Wilt and root rot

Sclerotinia sclerotiorum

Gram/Chickpea

Wilt, blight, seed and root rot

F. orthoceros var ciceri, F. solani, F. moniliforme and F. equiseti and Verticillium albo-atrum

Mung, Moth, Mash, potato & tomato

Wilt and root rot

F. solani and other species of Fusarium

Lentil

Wilt and root rot

F. oxysporum f. sp. lentis and F. solani

Seedling rot

F. moniliforme and Verticillium spp.

Pea

Wilt

F. oxysporum f. sp. pisi

Guar

Wilt and root rot

F. solani, F. semitectum and F. redicola

Potato

Wilt, root or dry tuber rot

F. oxysporum, F. redicola, F. angustum and F. solani

Chilies

Damping off, wilt and root rot

Fusarium spp.

Tomato

Wilt, root rot and damping off

F. oxysporum f. sp. lycopersici and F. solani

Cucurbits e.g. melons, gourds etc.

Wilt and root rot

Fusarium spp.

Coriander

Seed infection

F. moniliforme

Citrus e.g. orange, lemon etc.

Wilt and root rot

F. solani and other species of Fusarium

Banana

Wilt/ Panama disease

F. oxysporum and F. moniliforme

Mango

Wilt and root rot

Fusarium spp.

Malformation

F. equiseti, F. moniliforme and other species of Fusarium

Apple

Wilt and root rot

F. solani

Guava

Wilt and root rot

F. solani and other species of Fusarium

Shisham

Wilt and root rot

Fusarium and Verticillium spp.

Forest trees

Wilt and root rot

F. oxysporum, F. solani and other species of Fusarium and  Verticillium

 


 

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