Forewarning Rice Blast in India

Forewarning Rice Blast in India
(Pramanick et al., 1990), hills of Uttar Pradesh (Bhatt, 1992) and Nagaland (Sharma
et al., 1993), though the sowing months varied from location to location depending
up on the elevation, etc., at the respective locations.
Results of kharif, 2001 indicated that blast severity was high in the late sown
crop, when compared to the early sown crop. The average leaf blast infection in the
three different sowings was 58.8, 63.1 and 80.1 %, respectively. At Rajendranagar,
the average daily weather conditions of a week earlier to the date of observation of
high blast severity (17 th Oct.2001) indicated that the minimum temperature (T Min ) was
22.2 o C and maximum (T Max ) was 29.8 o C , with a rainfall (Rf) of 27.4 mm and 4 rainy
days with in a week (R W ), with morning Rh (Rh M ) of 92 % and evening Rh (Rh E ) of 58
%, were found congenial for blast disease development. Neck blast infection averages
in three different sowings were 57.0, 49.9 and 59.7 %, respectively. Before a week
earlier to the neck blast observation (24 th Oct. 2001), the daily averages of weather
conditions were: T Min 21 o C , T Max . 30.4 o C , Rf 31.1mm, 2 rainy days with in a week,
morning Rh of 91 % and evening Rh of 61 %. It is possible that these weather conditions
might have favoured the increase in the neck infection.
Analysis of daily weather variables of 2002 during preceding week (Table 6)
of high disease intensity showed that the following weather conditions were found
favourable. T Max : 30.2 0 C, T Min : 21.7 0 C. Rh M : 99.9 %, Rh E : 64.6 %, Rf: 0.03 mm and
R W : 1. All these conditions led to the epidemic progress of leaf blast to the extent of
10.26 % (Fig. 4), and the relationship was best explained by the following equation
Y = 43.287 - 2.859** T Min + 0.354** Rh E + 1.635** Lw . The combined effects of T Max :
28.7 0 C, T Min : 15.0 0 C. Rh M : 98.3 %, Rh E : 40.6 % resulted in significant effect on neck
blast severity (10.84 %), and this relationship between neck blast incidence and the
weather parameters resulted in the following equation: Y = 128.885 -1.968** T Min -
0. Rh M + 0.18* Rh E + 1.499*L w . Observations made by Bhatt (1992) revealed that
minimum temperature between 15-20 0 C, daily average temperature between
22-25 0 C and the difference between maximum and minimum temperature between
10-15 0 C, more days with Rh 90 % or above, higher rainfall and more number of
rainy days appeared important factors for the development of blast.
During 2003, weather in general was more favourable for the disease
development compared to kharif, 2002, which was officially acknowledged as “the
first-ever all-India drought year” since 1987. Analysis of daily weather variables of a
week earlier (Table 7) to the high disease intensity in 2003 were T Max : 29.3 0 C, T Min :
22.3 0 C. Rh M : 100%, Rh E : 80.1 %, Rf: 5.3 mm and R W : 6. These conditions led to the
high leaf blast severity (77.1 to 92.9 %) at different sowings (Fig. 4), the relationship
of which was shown in the equation as: Y = -1537.742 + 13.589** T Max + 13.413**
T Min - 6.197** R W + 9.431** Rh M + 1.758 L W . Neck blast was also maximum (26.3 to
34.7 %) in kharif, 2003, with favourable weather conditions, viz., T Max : 29.5 0 C, T Min :
18.2 0 C. Rh M : 100 %, Rh E : 68.3 %, Rf: 1.6 mm and R W : 5. However, the regression
equation (Y = - 155.37 + 8.493** T Max - 8.061** T Min + 3.82* Rw + 0.924 Rh E )
18