Spatial distribution of chili leaf curl disease and its correlation with environmental factors in Southern Irrigated Plain Zone of Pakistan

Chili leaf curl virus (Genus: Begomovirus, Family: Geminiviridae) a whitefly transmitted virus causing chili leaf curl disease (ChiLCD) occurs in Pakistan. Spatial distribution of ChiLCD and its correlation with environmental factors in southern irrigated plain zone of Pakistan was determined. The incidence and severity was recorded based on biological symptoms. There were five repeats for observation at each site; individual repeat comprised ten chili plants. Incidence of the disease was calculated by using mathematical equation, 0-5 rating scale was used to record the disease severity. Metrological data was collected from nearby regional agro metrological stations. There was significant variation among the disease incidence recorded at different sites of Hyderabad (df= 9, F= 13.5, P= 0.0000), Tando Allahyar (df= 9, F= 1.79, P= 0.1037), Mirpur Khas (df= 6, F= 1.74, P= 0.1555), Jamshoro (df= 5, F= 3.81, P= 0.0138), Larkana (df= 8, F= 9.99, P= 0.0000) and Dadu (df=9, F=3.93, 0.0015). Severity of the disease was ranging between 1 to 2-score in the surveyed areas. The minimum and maximum temperature were significantly related with the disease incidence (r= 0.7229, P= 0.0458 and r= 0.8073, P= 0.0218, respectively) but there was non-significant relation with severity of the disease. Incidence of the disease was negatively correlated with relative humidity (r= -0.8073, P= 0.0218). There was non-significant negative relationship of the disease severity with relative humidity and rainfall (r= -0.5774, P= 0.1511, r= -0.3515, P= 0.3894, respectively). Rainfall was also negatively non-significantly correlated with the disease incidence.


Chili
(Capsicum annuum L.) is economically-important cash crop of Pakistan. Chili is originated from Mexico more than 8,000 years ago, and their domestication and cultivation for the first time in Meso-America occurred approximately 6,000 years ago [1]. It is commonly grown in several subtropical and tropical countries areas of Europe and Asia and is used in both food and medicine. In Pakistan it is grown in different agro ecological regions during summer and winter seasons [2]. Chili is a versatile crop in which fruits are harvested at either the mature green or red stage of maturity. It is also commonly called as red pepper. Chili is also high consumed in tropical and subtropical areas due to its nutritional value and considered as cash crop for farmers and in developed and developing countries [3]. The chili prefers warm humid climate soils, which are basically light and wall drained are best suited. Transplanting has to be done when the plant reaches the height of 15 to 20 cm. it is always done in to pair. Maintaining spacing between rows is essential. Weed free conditions should be maintained. Irrigation must be provided every for up to 15 days after transplanting. The crop maturation period is about 150-180 days which is dependent on variety, climate, fertility and water management. The growth of chili consists of vegetative and reproductive phases. Vegetative phase in chili extends to 75-85 days followed by 75-95 days of reproductive phase [4]. Approximately (Hameed, 1995). Chili leaf curl virus (Genus: Begomovirus, Family: Geminiviridae) a whitefly transmitted virus causing chili leaf curl disease (ChiLCD) which is also distributed throughout the world [7-9]. ChiLCD is the most destructive disease in terms of incidence and yield loss. In severe cases, 100 percent losses of marketable fruit have been reported [10,11]. The typical symptoms consisting of leaf curling, rolling and puckering; blistering of intravenous areas, thickening and swelling of the veins, shortening of internodes and petioles, crowding of leaves and stunting of the whole plant [12]. The most effective method of viral disease management is the use of disease resistant plant varieties. For such purpose different chili varieties have been screened against the leaf curl virus in greenhouse to examine most tolerant variety for recommending farmers [13][14][15]. However, there is no any baseline work conducted in recent to assess high disease risk points for such screening and its correlation with environmental conditions is also not known during current decade. Thus, this study is planned to determine spatial distribution of ChiLCD and its correlation with environmental conditions in Indus delta and southern irrigated plain zone of Pakistan.

Materials and methods
The disease assessment Visually assessment method was used to record intensity of the ChiLCD under natural conditions at commercial farmer fields in various districts of Sindh province viz., Larkana, Dadu, Mirpurkhas, Tando Allahyar, Hyderabad and Jamshoro. Fields were selected randomly with the keenness of growers for the survey to cover the main arable areas which were managed by growers. There were five repeats for observations at each site, individual repeat was comprising ten chili plants. Incidence of the disease was calculated by following mathematical equation;

Incidence (%) = No. of symptomatic plants × 100 Total Number of plants under observation
Disease severity was measured by using 0-5 rating scale previously described by [10].

Metrological data
Metrological data was collected from nearby regional agro metrological stations.

Statistical analysis
Data collected on the disease intensity measured as disease incidence was processed in analytical software (STATISTIX v. 8 (Fig. 3). In district Jamshoro, 100 % incidence of the disease was recorded at Village Nawab Chohan and Allah Warayo Khoso-Farm (Fig. 4). Field at Village Sikandar Chohan in Jamshoro showed 92 % incidence of the disease which was higher than Near Kotri Junction (90 %), Saleem Khoso-Farm and Lal Bux-Farm (86 %) (Fig.  4). In district Larkana nine fields were surveyed for recording the disease. Significantly higher disease incidence 76 % was observed at field of Mukhtair Unar-Farm followed by Village Traghar (64 %) (Fig. 5).

Relationship of chili leaf curl disease intensity and environmental factors
Relationship of chili leaf curl disease intensity with environmental factors such as minimum and maximum temperature (℃), relative humidity (%) and rainfall was determined. The minimum temperature showed significant relation (r 2 = 0.7229, P= 0.0458) with the disease incidence (%), where as it was non-significantly (r 2 = 0.4124, P= 0.2992) related with the disease severity (Table 1). Similarly, the maximum temperature was significantly (r 2 = 0.8073, P= 0.0218) correlated with the disease incidence but non-significantly (r 2 = 0.5774, P= 0.1511) correlated with the disease severity ( Table 1). The disease incidence and relative humidity showed significant negative relationship (r 2 = -0.8073, P= 0.0218) ( Table 1). There was non-significant negative relationship of the disease severity with relative humidity (r 2 = -0.5774, P= 0.1511) and rainfall (r 2 = -0.3515, P= 0.3894) ( Table 1). Whereas rainfall was also negatively non-significant correlated with the disease incidence (r 2 = -0.2953, P= 0.3894) ( Table 1).

Discussion
This study was conducted to determine spatial distribution of chili leaf curl disease and its vector in southern irrigated plain zone of Pakistan. The disease was recorded in term of incidence and severity based on biological symptoms in different districts viz., Larkana, Dadu, Mirpurkhas, Tando Allahyar, Hyderabad and Jamshoro. Vector population was also scouted. Additionaly impact of environmental factors on the disease development was analysis. There was significant variation among the disease incidence recorded at Hyderabad (df= 9, F= 13.5, P= 0.0000), Tando