Effect of leaf temperature on Alternaria blight in wheat-mustard intercropping system

Macroclimatic parameters viz. maximum and minimum temperature, relative humidity, cloudiness, rainfall and bright sunshine hours affect the development of disease, particularly Alternaria leaf spot in Brassica. Besides microclimate, leaf physiological conditions, viz leaf temperature, stomatal diffusion resistance and transpiration rate of mustard leaf have a significant contribution in the development of Alternaria leaf spot caused by Alternaria brassicicola (Chakraborty, 1993). Intercropping of groundnut with bajra, sorghum, pigeon pea and maize in the ratio of 3:1 significantly reduced the incidence of peanut bud necrosis disease (Sunkad et al, 2005). But no information is available, whether intercropping of mustard with a cereal like wheat affects the development of Alternaria leaf spot in mustard. To have an understanding on the development of Alternaria leaf spot in mustard under intercropping, an experiment was conducted for two consecutive years (2008-09 and 2009-10) in which leaf temperaturean important leaf physiological condition of mustard was measured under mustard-wheat intercropping system and correlated with the disease occurrence.

Macroclimatic parameters viz. maximum and minimum temperature, relative humidity, cloudiness, rainfall and bright sunshine hours affect the development of disease, particularly Alternaria leaf spot in Brassica. Besides microclimate, leaf physiological conditions, viz leaf temperature, stomatal diffusion resistance and transpiration rate of mustard leaf have a significant contribution in the development of Alternaria leaf spot caused by Alternaria brassicicola (Chakraborty, 1993). Intercropping of groundnut with bajra, sorghum, pigeon pea and maize in the ratio of 3:1 significantly reduced the incidence of peanut bud necrosis disease (Sunkad et al, 2005). But no information is available, whether intercropping of mustard with a cereal like wheat affects the development of Alternaria leaf spot in mustard. To have an understanding on the development of Alternaria leaf spot in mustard under intercropping, an experiment was conducted for two consecutive years (2008-09 and 2009-10) in which leaf temperature-an important leaf physiological condition of mustard was measured under mustard-wheat intercropping system and correlated with the disease occurrence.
Twenty five plants were selected from each plot and labeled. Five leaves each from upper, middle and lower tiers of the mustard canopy were selected and thermocouples were placed on each leaf. The leaf temperature was measured in voltage output. The leaf temperature was measured on 14 th December(D 1 ),21 st December(D 2 ), 28 th December(D 3 ),5 th January(D 4 ),12 th January(D 5 ) and 19 th January(D 6 )(2008-09 and 2009-10) with simultaneous measurement of Alternaria spot per leaf were counted. The disease data were recorded from twenty five randomly tagged plants/plot on the basis of 1-9 scoring scale and Percent Disease Index (PDI) was computed on the basis of the recorded data using the formula developed by Wheeler (1969)  The data were analyzed statistically for interpretation of results. The PDI data have been represented to the nearest whole number in the tables. But the angular transformation values with which the statistical analyses were done were accommodated after the decimal place.
The Alternaria leaf spot appeared on 14 th December (both years) when leaf temperature was above 23 0 C. It was observed that the temperature of leaf increased gradually from upper to lower tier of the canopy in all the treatment combination, the sole mustard recorded the maximum leaf temperature and the PDI also increased in the same way. Among the different treatment combinations, the sole mustard recorded the maximum leaf temperature and PDI. As the number of wheat rows increased, the leaf temperature in mustard decreased and also the PDI values. The six wheat: two mustard row ratio recorded the lowest leaf temperature and the PDI values (Fig. 1). Significant differences in leaf temperature as well as PDI values were Effect of leaf temperature on Alternaria blight in wheat-mustard intercropping system On the 3 rd date (28 th December), leaf temperature again showed an increasing trend and the PDI values were also increased. On the 4 th and 5 th dates (5 th January and 12 th January), leaf temperatures again recorded a reduction with the no alteration in disease spread. On the 6 th date (19 th January) leaf temperature of mustard crop increased sharply with the concomitant increase in PDI values; the lower and mid tier leaves were highly affected by the Alternaria leaf spot.
On all the dates of observations, leaf temperature and PDI values were significantly higher in sole mustard stand and the lowest values were obtained in case of six wheat and two mustard row ratios. As one goes from upper to lower tiers, leaf temperature increased significantly with the concomitant increase in the number of Alternaria leaf spots , leading to the maximum infection in the lowermost tier of the crop canopy.
The correlation studies showed that significant correlation did exist in between leaf temperature and PDI of Alternaria leaf spot in all the tiers of the canopy throughout the period of disease infestation (Table 1). This shows that the temperature is one of the key factors for the occurrence as well as spread of Alternaria brassicicola. As the pathogen infects a host, the host, according to the general principle of immunity, tries to defend itself from the pathogen (Chakraborty, 1993). The spot development is the ultimate expression of pathogen domination where the leaf resistance is over. The increase of leaf temperature in infected leaves might be due to biochemical defensive mechanism where ATP had to be utilized for resistance (Chakraborty, 1993). The upper tier leaves were subjected to wind flow and thus abled to transpire which cooled the leaf tissues in comparison to middle or lower tier leaves. Chakraborty (1993) also observed that increased leaf temperature in mustard caused an increase in stomatal diffusion resistance with the resultant decrease  in transpiration rate. Among the intercropping treatments, it was observed that leaf temperature decreased with the increase in the number of wheat rows. This might be due to different geometric arrangement of wheat leaves which invited the variation in radiation receipt and wind flow within the canopy. The results clearly showed that intercropping reduced the occurrence of Alternaria leaf spot in mustard and six wheat: two mustard was found to be the best row ratio in the context of Alternaria leaf spot development. Sunkad et al (2005) also observed that intercrops acted as barrier for the movement of insect vectors from one place to another thus controlled the viral disease. These results also indicated that the leaf temperature played a regulatory role in the development of Alternaria leaf spot, when the leaf temperature increased above 23 0 C disease appeared.