June 05, 2006	Monday	Jumadi-ul-Awwal 8, 1427

OKRA or lady finger requires high soil temperatures and day/night temperatures for best production. Soil types, preparation method, time, sowing, seed quality, irrigation, fertilizer applications, inter-culturing etc., play a pivotal role, along with the insect pest and the diseases.

The diseases and insect pests vary from year to year. Their presence, incidence and severity depend up on susceptibility and on environmental conditions.

The Integrated Plant Disease Management (IPDM) is less expensive and easy to adopt. Ratoon crop is not suggested in heavily infested crop field but in case of ratooning, proper irrigation and fertilizer applications along with inter-culturing, weed and pest management practices must be ensured.

Okra plants are attacked by a number of seed and soil borne diseases caused by different fungi. Root or collar rot and damping-off disease caused by Macrophomina phaseolina (Rhizoctonia bataticola), R. solani, Fursarium solani and Pythium butleri or root and stem rot caused by Phytophthora palmivora, spread rapidly under optimum conditions and is considered a major problem.

Root rot is one of the most destructive diseases, its incidence ranges between 10-80 per cent with 55-80 per cent in the crop grown on small scale as kitchen/home gardening, and 10-45 per cent in the crop sown on large scale under field conditions.

The fungi reproduces the diseases due to its presence in soil and seed or with the infected plants. Naturally, infected seeds appear brown to black and show die-back and root rot. The affected plants show dark brown to black discoloration from the base of stems. Root rot may be found on individual plants or infected plants, scattered or in groups and patches.

The crop grown on ridges in field or cultivated on flat beds as kitchen/home gardening is affected by these diseases. Severely infected plants become dead, their roots turn dark brown. As yet none of the researchers could find a completely disease resistant variety.

Some have recommended chemical seed or soil treatment while others evaluated leaf extracts and oil cakes. According to report, the incidence of five seed-borne diseases (viz. foot and root rot, anthracnose and die-back, cercospora leaf spot, corynespora leaf spot and leaf blight are found to be reduced by the use of seeds treated with fungicides and clean apparently healthy seeds which also increases the seed yield by 21.62 and 15.31 per cent, respectively, over the unclean seeds.

Experiments on effect of mulching methods on seed and soil borne diseases revealed that the straw mulching method is more effective on soil moisture conservation, resulting in vigorous plant growth and development with less disease incidences.

The straw mulching method yields more with few root rot incidences over plastic mulching and weed management practices. Mulching with transparent polyethylene sheet combined with garlic or onion aqueous extracts for 30 days caused a highly significant reduction in the percentage of diseased plants and severity in the crop sown in winter or cool season.

Growers are advised to include plant extracts and mulch instead of fungicide application against seed and soil borne diseases as pollution free crop production technology produces disease and poison free vegetables.

Yellow vein mosaic virus (OYVMV) disease is transmitted by insect vector, the whitefly, Bemisia tabaci. The most susceptible stage of the crop is from 35 to 50 DAS. The initial symptom on young leaves is a diffuse, mottled appearance. Older leaves have irregular yellow areas which are interveinal. Clearing of small veins starts near the leaf margins, at various points, about 15 to 20 days after infection. Thereafter, the vein clearing develops into a vein chlorosis.

The newly developed leaves exhibit an interwoven network of yellow vein, which encloses green patches of the leaf. Fruit developed on infected plants has irregular yellow areas which follows a longitudinal alignment. The fruits are also malformed and reduced in size. The fruits are mostly yellow, small, though and fibrous.

Correcting nutrient deficiencies is very important as plants become week and susceptible to insect pests and diseases. Evaluation of soil fertility (for estimating initial nutrient level) and application of required nutrient in the shape of synthetic fertilizers, manures etc., with correct time, dose and method improve resistance against insect pests and diseases. Injudicious and excess use of nitrogenous fertilizers (urea) makes feasibilities/possibilities to increase insect pest population and susceptibility to the diseases.

The population of whitefly must be reduced to obtain good harvest as systemic insecticides have long residual effect. Plant extracts and other less hazardous products may be used instead of pesticides.

The cultural practices such as discouraging cultivation of okra in and around cotton fields; intercropping with non-host crops; collecting and burning diseased okra/bhindi plants; destroying wild host plants, cultivating whitefly resistant varieties should be preferred over others.

Okra leaf curl disease (OLCD) is suspected of being associated with a whitefly-transmitted geminivirus (genus begomovirus). The disease becomes major whenever vector is active. The disease is characterized by either upward or downward leaf curl accompanied by small and main vein thickenings (SVT & MVT) on leaves, pronounced on underside and stunted plant growth.

At a later stage, all diseased leaves develop enations which become prominent with time. Similar Integrated Plant Disease Management (IPDM) measures are suggested as recommended against the yellow vein mosaic virus.

The powdery mildew (erysiphe cichoracearum) fungus coats the upper and lower leaf surfaces with a white film of mycelium. Initially white superficial spots appear on leaves but the entire surface may cover with powdery mass. Diseased parts turn brown and the affected leaves start drying. If conditions remain ideal for disease development, defoliation also takes place, plant growth and fruit development reduces the yield.

Normal and judicious irrigation and fertilizer application followed by sanitation through inter-culturing for control of alternate host plants (weeds) is recommended. In case of severity, different foliar fungicides could also be used.

There are several fungi (Cercospora abelmoschii or C. malayensis, Alternaria sp., Ascochyta sp., etc., causing leaf spot diseases. Most of these are seed and soil borne or can also survive on plant debris and attack crops season after season. The leaves, stems or roots develop small, yellowish-green to brown spots. In some cases, the old leaves may also show water-soaked spots. The fruit develops small, raised rough spots or rot spots, or fruit may fail to set. None of leaf spot have shown to cause economic loss. No control is recommended. However, precautionary measures include the use of disease-free seed, crop rotation, and avoidance of overhead watering.

Okra is highly susceptible to root-knot nematodes, meloidogyne species. The above ground symptoms are similar to those described for root rot and wilt diseases. Only the difference is appearance of root galls/knots of different sizes, instead of root rotting. The infected roots also become enlarged and distorted. Root-knot nematode has wide host range. Weed management, crop rotation and intercropping or mix cropping or cover cropping with non host is recommended.

Suggestions:

* Well drained-sandy loams (high in organic matter) are desirable because poorly drained soils may result in drowning of plants and getting good stands on heavy clays also becomes difficult.

* Okra grows best in neutral to slightly alkaline soils with 6.5, the optimum pH.

* Overnight soaking of seeds speeds up germination.

* Seeds are generally sown at a depth of one-half to one inch which may require from one to six kg/per acre.

* Tall or spiny cultivars require wide and dwarf varieties need narrow spacing of about 12 to 15 inches from plant to plant and 36 to 42 inches from row to row.

* To establish good stands, initially, three to four seeds per point are planted but after few days the week plants are removed by thinning.

* Over-dosage of nitrogen can cause excessive vigour that attracts insect pests, favours pathogens to cause diseases and poor yield. Therefore, soil testing is necessary. Addition of manure or other organic material is usually beneficial in poor and light-textured soils.

It is also suggested that researchers must conduct various experiments on other aspects of non-chemical and bio-control methods etc., against diseases to reduce health hazards and environmental pollution. The growers can also conduct small scale trials with minimum expenses for solving their problems by their own.