IPM strategies for Burewala Strain of Cotton Virus (BSCV)
Muhammad Amjad Ali
Cotton, the backbone of Pakistan’s agriculture and economy is at risk due to leaf curl virus disease. Never has a single pathogen or insect pest threatened Pakistan’s cotton culture, as has the leaf curl virus. Numerous strategies could be adopted to combat this disease.
Leaf curl disease is caused by a Gemini group of viruses and is transmitted by whitefly (Bemisia tabaci Genn.). In Pakistan, cotton leaf curl virus (CLCuV) was first observed on stray plants in 1967 but appeared at epidemic level for the first time in 1989 over a limited area. CLCuV, as a disease, was overlooked until 1992 when it devastated cotton production in the Punjab (Pakistan). In 1993-94, about 0.89 million hectares were badly damaged resulting about two million bales loss in production due to CLCuV. In economic term, the country had suffered a loss of about 7.6 million bales, which costs to the tune of Rs. 71 billion since 1988 due to the infestation of CLCuV. Yield decreased from 1.938 million metric tons in 1991 to 1.445 million metric tons in 1992 and fell further to 1.105 million metric tons in 1993.
The CLCuV was the main force behind yield decline in these years. The first 3 years of the disease epidemic (1992-1994) in Pakistani Punjab were the most severe in terms of disease intensity. An International Cotton Advisory Committee’s (ICAC) analysis suggested that 25% of the increase in 1993/1994 season average of the Cotlook-A Index was due to lower production in Pakistan (ICAC, 1994). The epidemic of CLCuV in Pakistan is one of the best examples of the dramatic shift in importance of a previously unimportant endemic disease. Cotton in Pakistan is cultivated in two provinces, Punjab and Sindh. The Punjab Province that contributes about 78% of the area and production has yet not recovered from the virus shock. The CLCuV has crossed to the bordering Indian Punjab with more than 500,000 ha affected during 1999-2000 (ICAC, 1999).
Symptoms of the disease as observed in Pakistan begin with the thickening of small veins visible on the lower surface of the upper young leaves. Under severe conditions, leaves curl downwards or upwards and plants become stunted due to shortening of inter-nodal distance. Losses due to CLCuV depend upon time and severity of infection. Appearance of the disease at the seedling stage seriously hampers flowering, boll formation, and maturation thus reduces seed cotton yield and fiber quality. Although cotton cultivars with field resistance to CLCuV have been developed by cotton breeders in Pakistan and are under cultivation but they have narrow adaptability and yield less than otherwise susceptible or tolerant cultivars. The onset of the disease was to some extent checked by various prevailing tolerant cultivars in the past.
But recently, cotton leaf curl virus has again emerged as a key disease in the province of the Punjab in general and Burewala area in particular. This re-emergence of virus commonly called as Burewala Strain of Cotton Virus (BSCV) is much more hazardous version than the previous one and could develop into a serious problem. It is suspected that previous virus strains have mutated and resistant varieties are showing symptoms of CLCuV in the whole cotton belt of Punjab.
Even the parent material (LRA-5166, CP-15/2, and Cedex) used to develop cotton leaf curl virus resistant varieties have also been found susceptible in Burewala territory. The Burewala virus disease has infested the most productive areas of southern Punjab (Khanewal, Multan Lodhran and Vehari districts) and is expected to lower production considerably in the country during 2008/09. The infestation of CLCuV disease has crossed the boundaries of the Punjab and entered into the neighboring provinces with especial reference to Sindh (ICAC, 2006).
The constant use of CLCuV susceptible varieties without any program of their replacement with the resistant blood constitutes a major risk for cotton production in Pakistan. So a premier focus should be given to eliminate the CLCuV disease and a well-planned program of evolution and introduction of CLCuV resistant varieties of desired characteristics must be in place to gradually replace the existing CLCuV susceptible varieties. This is the sole, the most promising and the least expensive method of disease suppression.
Integrated Disease Management (IDM) can be carried out to suppress this dangerous disease. IDM is actually a combination of various cultural and chemical control mechanisms in addition to protection through natural predators and parasites. There are various other management strategies to combat CLCuV in addition to resistant cultivars. Weeds in and around cotton fields should be removed carefully and cotton fields should be kept clean at all the times. Spring crops should be sown and harvested on time in the cotton belt. If spring crops are harvested late, close to the cotton planting season, viruses and insects will be transferred to the cotton crop. The recommended interval between harvesting of spring crops and the sowing of the cotton crop should be at least one month.
Natural protection may also be provided to the crop through biological control by increasing the population of beneficial insects such as chrysopa, ladybird beetle, trichograma and spider. The previous year’s cotton stubs must be removed from the fields because sprouts from diseased plant stubs transmit the disease. Early sowing should not be done as it increases the population of whitefly, which acts as a vector for the virus. Cotton should not be sown before 15th April under any circumstances. Seed must be treated with systemic insecticides before sowing that helps to control whitefly and other sucking insects. Intercropping of cotton in orchards and planting of alternate hosts of cotton pests (like okra, sunkukra and sesame) on the borders of cotton fields should be avoided to keep the disease distant from the cotton crop.
The cotton crop should not be over-nourished. Apply only the recommended quantities of fertilizers and irrigation water. Thinning should be done at the proper time to remove weak and virus-affected plants. The crop must be protected vigilantly against sucking insects especially whitefly which is the main vector in the spread of the virus from plant to plant or from one crop to another. The recommended threshold for whitefly in Pakistan is five adults or nymphs or combination of both. However, it has been shown that even a single whitefly/leaf is enough to transmit the disease. Whitefly is an established pest in Pakistan and it is impossible to eliminate the entire whitefly population, however, the expert advice is that restricting the whitefly population to a minimum is helpful to reduce the chances of CLCuV infestation. This is why farmers are specifically advised to keep a strict watch on the whitefly population. The following measures have been recommended to minimize the whitefly population in order to limit the danger of Burewala virus infestation.
• Use yellow sticky traps during the earlier part of the cotton season to attract whiteflies.
• Destroy old diseased leaves to limit the population of whitefly adults and nymphs.
• Over dosing of nitrogen (beyond recommended levels) or under-dosing of phosphorus and magnesium may increase the whitefly population. Use these fertilizers judiciously and strictly as recommended.
• Pressure spray the crop with water on three consecutive days to destroy whitefly adults.
• Spray insect growth regulators on the underside of leaves to destroy whitefly nymphs.
It concludes that the re-emergence of CLCuV in Southern Punjab may cause devastating effects on cotton production which ultimately will affect the country’s economy. So in addition to the evolution of resistant cultivars various managing practices as mentioned above should be kept under consideration for the effective control of the disease.