3.6.3S
A CYCLIC DISEASE AND THE EMERGENCE OF A NEW DISEASE IN LATIN AMERICA REAFFIRM THE NEED FOR INTEGRATED PEST AND DISEASE MANAGEMENT STRATEGIES IN RICE

L CALVERT, FM MORALES and FJ CORREA-VICTORIA

CIAT, A.A. 6713, Cali, Colombia

Rice hoja blanca virus (RHBV) epidemics are cyclic occurring every 8-15 years. During the peak of epidemics, yield losses at the country level have been estimated to be between 25% and 50%. Venezuela, Peru, Costa Rica, Dominican Republic, Surinam and Colombia are currently experiencing outbreaks of RHBV. The vector of RHBV is the planthopper Tagosodes orizicolus (Muir). The planthopper can cause direct damage to rice. When the threat of RHBV is present, rice producers use up to ten sprays to control the vector and this leads to the emergence of pesticide resistant populations. The use of broad-spectrum pesticides also leads to a resurgence of planthopper populations. Integrated pest management strategies must complement the use of resistant varieties to control the losses caused by RHBV [1].

A new disease of rice, known as "entorchamiento" (crinkling), was first noticed in the Eastern Plains of Colombia, in 1991. Symptoms include seedling death, foliar striping and severe plant malformation. Tissue extracts and partially purified preparations from diseased rice plants, contained virus-like particles ca. 20 nm in diameter, with a bimodal length of 260 and 360 nm. Particle aggregates were also observed in the cytoplasm of infected leaf cells. Electrophoretic analyses of purified preparations and ds-RNA extracts, revealed a single protein species of M, 22,500, and four ds-RNA bands ca. 6,300, 4,600, 2,700 and 1,800 bp in size. Cystosori, characteristic of plasmodiophorid fungal vectors of plant viruses, were consistently observed in the roots of diseased rice plants. Planting rice in soil collected from affected fields reproduced the crinkling symptoms. The "entorchamiento" disease of rice in Colombia, is identical to "rice stripe necrosis" described in West Africa, in 1977. Rice stripe necrosis is caused by a furovirus (RSNV) transmitted by the fungus Polymyxa graminis. A serological assay with RSNV antiserum confirmed the emergence of rice stripe necrosis in the Americas [2].

In an evaluation of 63 commercial rice cultivars, the rice cultivars Makalioka and Mudgo were highly resistant to RSNV. Two rice lines (CT 9901-3-2-M-3-MI and CT 11026 -3-9-1T-2P-2P-2) did not exhibit any symptom while in total fourteen lines exhibited 20% or less incidence of RSNV. Since the roots of resistant cultivars like Makalioka contained large number of cystosori, resistance appears to be to the virus. In addition to resistant varieties, the disease can be controlled by cultural practices. Rotation can be one method of mitigating losses in fields where the disease is endemic. For both diseases, plant resistance is only one component of the control strategies. Proper management practices can help prevent the spread and the epidemics of both diseases.

References
1. Lentini Z, Calvert L, Taberes E, Lozano I, Cuervo M, Roca W, Ramirez BC, 1997. General Meeting of the International Program of Rice Biotechnology. Abstracts pp. 61.
2. Morales FJ, Arroyave JA, Velasco AC, Castaho M, 1994. Fitopatologia Colombiana 19, 48-54. (in Spanish).