Foliar blights of wheat in India: Germplasm improvement and future challenges for sustainable, high yielding wheat production
1998
Nagarajan, S. | Kumar, J. (Indian Council of Agriculturral Research, Karnal (India). Directorate of Wheat Research)
Spring wheat is grown over 25 million ha in India, covering six mega environments as a winter crop between November to April. Production is on the order of 65.0 million t. In the northeastern plain zone (NEPZ), a major part of the 9.5 million ha is subject to severe leaf blight development where rice precedes wheat; conditions are warm and humid, nutrient application is poor, and soils are flooded and saline. Since varietal replacement is slow, brown rust (Puccinia recondita tritici) severity tends to be high and, when coupled with leaf blight, crop losses are very high. Leaf blight is a disease complex caused by a number of pathogens, with the most serious being Bipolaris sorokiniana (syn. Helminthosporium sativa) that causes spot blotch. This pathogen is capable of causing damage from the primary leaf stage, though the plant tends to become more susceptible after flowering. The small spots on the leaf, if numerous, coalesce and the leaf prematurely dries up, reducing the photosynthetic area of the plant. The role of toxin in chlorosis and leaf drying (necrosis) is not well understood. It is further complicated by the slight differences in host response, the need for a scale to separate the more complex levels of host pathogen interaction, and by the wide host range of the pathogen. Bipolaris sorokiniana has been found to infect barley, oats, and rice, as well as 12 other grasses. At the Directorate of Wheat Research, a reliable technique has been developed to create artificial epiphytotics under a plastic tunnel and in the field. Also, a new rating procedure has been suggested to score the earhead, flag leaf (F), and F-1 leaf to enable plant breeders to select for more tolerant lines. In the northwest plain zone (NWPZ), tan spot (Pyrenophora tritici-repentis) is common on the lower leaves and leaf sheath of wheat plants, and spreads to upper leaves during prolonged wet and warm situations.
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