IPM Packages for Crops

diseases & viruses

DISEASES

Rice blast [ Magnaporthe oryzae/Pyricularia grisea/ Magnaporthe grisea ( Magnaporthales: Magnaporthaceae )] Rice blast is the most serious of the rice diseases because it infects all plant growth stages. This disease attacks all parts of the plant except the leaf sheath and spreads rapidly. Depending on the site of the symptoms, this disease is referred to as leaf blast, collar blast, node blast, and neck blast (or neck rot). It stunts or kills seedlings and tillering plants and also reduces the number of panicles and lowers grain weight and quality. Collar blast can also damage the entire leaf blade and the panicle. Sheath blight [ Rhizoctonia solani (Cantharellales: Cera- tobasidiaceae)] The characteristic symptoms of this disease are water-soaked, circular/ oblong/ ellipsoid/ovoid or even irregularly elongat - ed discolored lesions on the leaf sheath at or above the water level in lowland and at ground level in upland fields. This disease does not affect seedlings; however, under very high humidity and at favorable temperatures, even the seedlings may get wilted. No resistance variety is available. Brown spot [ Bipolaris oryzae ( Helminthosporium oryzae )] It is a seedborne disease. Leaf spots can be observed shortly after seedling emergence and continue to develop until maturity. Damage from the brown spot is particular- ly noticeable when the crop is produced in nutritionally deficient soil conditions and is indicative of a soil fertility problem.

Sheath rot [ Sarocladium oryzae (Hypocreales: Saro- cladiaceae) ] This disease develops irregular spots or lesions, with dark reddish-brown margins and gray center on leaf sheath, which caus- es discoloration of the sheath. Enlargement and coalescence of lesions often covers the entire leaf sheath, infected panicle gets rotten, and white powdery growth can also be seen. The disease appears late during the growing season of the rice crop and can cause yield losses from 20 to 85 percent. Seedling blight/Stem rot [ Sclerotium oryzae (Magnaporthales: Magnaporthaceae) ] This disease is most noticeable in rice fields during the latter stages of maturity. On leaf sheath, it initially causes black, rectangular lesions with distinct angular borders. At later stages, the lesions become larger and penetrate deep into the culm. Maximum injury to the stem occurs at harvest, resulting in lodging of plants and harvesting to become difficult. The pres - ence of tiny, black sclerotia at the base of the stem indicates its presence.

Bacterial blight [ Xanthomonas oryzae pv. oryzae ) (Xanthomonadales: Xanthomona- daceae)] This is a vascular disease resulting in systemic infection of rice. It causes tannish grey to white lesions along the veins. Al- though the first symptoms are observed at the tillering stage, the incidence increases with plant growth and peaks at the flower - ing stage. There are two different phases, the leaf blight phase and the kresek phase. In kresek stage, the leaves turn pale yellow and wilt; hence, it is the most destructive stage. In leaf blight phase, yellow lesions with wavy margins appear on leaf blades. In rainy seasons it can cause 50 percent yield loss. Resistant varieties play a major role in the management of this disease. Note: The majority of rice viruses occur in Asia (about 12 viruses) and most of them are transmitted by leaf hoppers and plant hoppers. Rice grassy stunt virus , Rice rag- ged stunt virus , and Rice Tungro are major issues in Asia. Rice Tungro (Rice tungro bacilliform virus and Rice tungro spherical virus) Leaf discoloration is a characteristic fea- ture of these viral diseases, which begins from the leaf tip and extends down to the lower leaf portion. Infected leaves also show mottled or striped appearance and stunting. Infection to rice plants reduces tillering, delays flowering, and can delay maturity. Panicles also become sterile or have partially filled grains and are covered with dark brown blotches. At the early crop growth stage this disease can cause 100 percent yield loss. Symptoms of the dis- ease are usually absent at the early growth stage. To manage this problem, use of resistant varieties is important. VIRUSES

Rice tungro, Rice Knowledge Bank-IRRI