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Infected leaves (IRRI)
Pathogen: Pyricularia oryzae Cavara (anamorph), Magnaporthe grisea (T. T. Hebert) Yaegashi & Udagawa (teleomorph)
The lesions are elongated and pointed at each end.
The pinhead-size brown lesions can be confused from the symptoms of brown spot disease caused by Helminthosporium oryzae.
The whiteheads symptom looks the same as the whiteheads produced by the stem borer.
In the tropics, blast spores are present in the air throughout the year, thus favoring continuous development of the disease. The infection brought about by the fungus damages upland rice severely than the irrigated rice. It rarely attacks the leaf sheaths. Primary infection starts where seed is sown densely in seedling boxes for mechanical transplanting.
In the temperate countries, it overseasons in infested crop residue or in seed.
Cloudy skies, frequent rain, and drizzles favor the development and severity of rice blast. High nitrogen levels, high relative humidity, and wet leaves encourage infection caused by the fungus. The rate of sporulation is highest with increasing relative humidity of 90% or higher. For leaf wetness, the optimum temperature for germination of the pathogen is 25-28 °C.
Growing rice in aerobic soil in wetlands where drought stress is prevalent also favors infection.
A fungus causes rice blast. Its conidiophores are produced in clusters from each stoma. They are rarely solitary with 2-4 septa. The basal area of the conidiophores is swollen and tapers toward the lighter apex.
The conidia of the fungus measure 20-22 x 10-12 µm. The conidia are 2-septate, translucent, and slightly darkened. They are obclavate and tapering at the apex. They are truncate or extended into a short tooth at the base.
Aside from the rice plant, the fungus also survives on Agropyron repens (L.) Gould, Agrostis palustris, A. tenuis, Alopecurus pratensis, Andropogon sp., Anthoxanthum odoratum, Arundo donax L., Avena byzantina, A. sterilis, A. sativa, Brachiaria mutica (Forssk.) Stapf, Bromus catharticus, B. inermis, B. sitchensis, Canna indica, Chikushichloa aquatica, Costus speciosus, Curcuma aromatica, Cynodon dactylon (L.) Pers., Cyperus rotundus L., C. compressus L., Dactylis glomerata, Digitaria sanguinalis (L.) Scop, Echinochloa crus-galli (L.) P. Beauv., Eleusine indica (L.) Gaertn., Eragrostis sp., Eremochloa ophiuroides, Eriochloa villosa, Festuca altaica, F. arundinacea, F. elatior, F. rubra, Fluminea sp., Glyceria leptolepis, Hierochloe odorata, Holcus lanatus, Hordeum vulgare, Hystrix patula, Leersia hexandra Sw., L. japonica, L. oryzoides, Lolium italicum, L. multiflorum, L. perenne, Muhlenbergia sp., Musa sapientum, Oplismenus undulatifolius (Ard.) Roem. & Schult., Panicum miliaceum L., P. ramosum (L.) Stapf, P. repens L., Pennisetum typhoides (L.) R. Br., Phalaris arundinacea L., P. canariensis, Phleum pratense, Poa annua L., P. trivialis, Saccharum officinarum, Secale cereale, Setaria italica (L.) P. Beauv., S. viridis (L.) P. Beauv., Sorghum vulgare, Stenotaphrum secundatum, Triticum aestivum, Zea mays L., Zingiber mioga, Z. officinale, and Zizania latifolia.
Conidia are produced on lesions on the rice plant about 6 days after inoculation. The production of spores increases with increase in the relative humidity. Most of the spores are produced and released during the night. After spore germination, infection follows. Infection tubes are formed from the appressoria and later the penetration through the cuticle and epidermis. After entering the cell, the infection tube forms a vesicle to give rise to hyphae. In the cell, the hyphae grew freely.
Rice blast infects the rice plant at any growth stage. Rice seedlings or plants at the tillering stage are often completely killed. Likewise, heavy infections on the panicles usually cause a loss in rice yields.
Rice blast is one of the most destructive diseases of rice because of its wide distribution and its destructiveness. In India, more than 266,000 tons of rice were lost, which was about 0.8% of their total yield. In Japan, the disease can infect about 865,000 hectares of rice fields. In the Philippines, many thousand hectares of rice fields suffer more than 50% yield losses. A 10% neck rot causes yield loss of 6% and 5% increases in chalky kernels.
There are some cultural practices that are recommended against the rice blast. For example, manipulation of planting time and fertilizer and water management is advised. Early sowing of seeds after the onset of the rainy season is more advisable than late-sown crops. Excessive use of fertilizer should be avoided as it increases the incidence of blast. Nitrogen should be applied in small increments at any time. Water management practices in rainfed areas lessen the likelihood of stress, which also aid in blast control.
Planting resistant varieties against the rice blast is the most practical and economical way of controlling rice blast.
Systemic fungicides such as pyroquilon and tricyclazone are possible chemicals for controlling the disease.
Suparyono, JLA Catindig, and IP Oña