Abstract: Cotton variety CIM- 240 was sown to determine the effect of cotton leaf curl viral disease (CLCV). The experiment was carried out near Chistian in District Bhawalnagar in 1997. Cotton sown in the orchard showed high CLCV incidence percentage as compared to cotton sown near the orchard and away from the orchard. Thus orchard plays a significant role in spreading the CLCV disease by providing favorable environmental condition for the multiplication of the vector of the disease as well as acts as a host of vector.
Introduction
Cotton, silver fiber is the most important cash crop of Pakistan. It is the main source of foreign exchange earnings, and considered to be the back bone of our economy. More than 53% Pakistan’s exports relate to cotton and its by product (Mukhtar et al., 1999). Insects pests contribute one of the major limiting factor in cotton production. Currently around 511 kg/ ha lint was produced and about 20 to 40 % loss is due to different pests of cotton in Pakistan (Ahmed, 1999).
For the last few years, the cotton whitefly, Bemisia tabaci Gennadius has become serious problem to cotton crop by reducing its yield directly and indirectly. The direct damage to cotton crop includes the sucking of cell sap and indirectly by the secretion of honey dew, which develop the sooty mold that lowers the photosynthetic activity of the crop, and transmitting Gemini virus also known as cotton leaf curl virus (Afzal et al., 2000). In Pakistan, the cotton leaf curl virus was first recorded from Multan in 1967 from a few plants (Hussain and Mehmood, 1988).
CLCV is largely responsible for the drop in cotton production from 12.8 million bales in 1991 to 1992, and to 8 million bales in 1993 to 1994, that is equivalent to a decrease in gross domestic product (GDP) 3 to 4 % (Anonymous, 1996-97 ). In 1995 the cotton whitefly caused a decline of 2.25 million bales of cotton by transmitting the Gemini virus in Pakistan (Anonymous, 1996). Leaf curl virus alone reduced the cotton production around 24% between 1992-96 in Pakistan (Poswal et al., 1998 ).
Keeping in view the serious activity of this pest an experiment was conducted to determine the effect of orchard for the spread of leaf curl disease in cotton crop.
Materials and Methods
The experiment was carried out at Markiz Shaher Farid near Chistian in District Bhawalnagar. Cotton variety CIM-240 was sown on a well prepared seed bed with single row cotton drill at a seed rate of 8 kg/acre in 1997. Randomized complete block design was employed with three treatments and each treatment was replicated 4 times. The samples were taken from the cotton which is sown in the orchard, near the orchard (20 meters from the orchard) and away from the orchard (100 meters from the orchard). The diseased effected plants were identified by direct observation. The CLCV incidence % was monitored weekly between August and September. A plant with around 33% damage or more was considered as a diseased plant. Randomly 100 plants were counted in single row and then calculated the incidence of the percentage of leaf curl disease. Data were calculated on weekly basis. Descriptive statistics and ANOVA models were calculated using the SPSS package (Ali et al., 2000).
Results
Cotton leaf curl virus incidence was frequently found on the cotton variety CIM-240 sown in the orchard as compared to the near and away from the orchard. A total of 2342 effected cotton plants by CLCV incidence was recorded direct observation of which 1699, 389 and 254 were recorded from cotton sown in the orchard, near the orchard and away from the orchard respectively. Mean CLCV incidence percentage 53, 12.1 and 7.9 % were calculated from the three treatments, respectively (Fig. 1).
The incidence of CLCV percentage was monitored weekly between August and September. Its incidence starts to develop in the month of August and continue to increase at its end (Fig. 2).
| Fig. 1: | Comparison the percentage of cotton leaf curl viral disease (Mean 1 standard error) within the treatments in 1997 |
| Fig. 2: | Weekly percentage of cotton leaf curl viral disease (Mean±1 standard error) in 1997. |
The CLCV incidence % reached at its peak in the last week of August in all treatments. Subsequently there was a gradual reduction by the end of September.
When the mean CLCV incidence percentage of three treatments were compared statistically highest %age was found on cotton sown in the orchard and it was significantly different from cotton sown near the orchard and away from the orchard (Table 1).
Non significant differences were observed on weekly basis of CLCV incidence percentage and replication, F3,96 = 0.128, P<0.996 and F3,96 =0.005 , P<0.999 respectively .
| Table 1: | ANOVA comparing the percentage incidence of CLCV disease on CIM-240 (1997) |
Discussion
All treatments are effected from leaf curl disease. However significant difference in CLCV incidence %age is found in the treatments. Cotton sown in the orchard shows highest CLCV incidence %age as compared to others treatments. This shows that vector population (Bemisia tabaci) was high in that area. A positive correlation between population size and spread of viruses has been reported (Saikia and Muniyappa, 1989). CLCV incidence %age was at its peak in the last week of August in all treatments. Our results are supported by Basu (1995) that the reproduction of whitefly was continued freely in cotton throughout the summer from August to September and dropping suddenly from September-October.
There are two reasons which enhance the vector population of whitefly in cotton sown in the orchard. Firstly, the host characteristics of the vector; secondly, the climatic conditions such as temperature, humidity prevailing in the orchard to enhance the vector population. Orchard plants (citrus) itself is a host of whitefly so, vector already exist there in the orchard, which is easily transfer on the cotton plants, that resulted in higher population on cotton crop sown in the orchard.
Orchard plants like Citrus, Guava and Jamen are the host of cotton whitefly (Basu, 1995). Whitefly has been noted as a pest under much higher temperature regimes. This might be due to favorable micro climatic condition offering protection against very hot macro climate. The microclimate of cotton sown in the orchard has favorable temperature not varied much as in cotton sown near the orchard and away from the orchard. Climatic conditions are perhaps the most important factors that influence population dynamics of whitefly which is responsible to spread of cotton leaf curl disease. This indicates the temperature range of 27-300C promotes rapid development of the whitefly (Gerling et al., 1986). High temperature and suitable nutritional status of host plant promote rapid development , leading to the exponential phase of growth. Exponential growth is usually followed by an outbreak of the pest population (Butter et al., 1985).
High humidity occurs in cotton sown in the orchard due to citrus plants canopy resulted in favorable humid conditions for whitefly multiplication. The high relative humidity (85%) play important role in regulating population of whitefly (Horowitz, 1986). Temperature and wind are the major environmental factors that affect activity of aerial vectors.
It is concluded that the orchard provide the favorable conditions for whitefly which effect to spread the cotton leaf curl viral disease.