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Brown spot, caused by C. miyabeanus, typically affects the leaves and glumes of the host plant, although seedlings, and the sheaths, stems and grains of adult plants may also be affected. Typical symptoms on leaves are evenly distributed oval-shaped lesions, up to 1 cm in length. The spots are brown, with greyish centres when fully developed. Young lesions, or lesions which develop under unfavourable conditions or on resistant host varieties, appear as small, dark-brown spots. Black or dark lesions with a velvety aspect also develop on glumes when conditions are favourable for sporulation. Lesions on the glume may lead to blackish spots on the endosperm. Injury to the grain may be associated with insect feeding by rice bugs (Leptocorisa oratorius) and several other fungi (Lee et al., 1986).
Most literature on the control of brown spot concerns the use of fungicides. Several methods have been considered to control the disease including management of soil nutrients, host-plant resistance and chemical control of both seeds and crop.
Cultural Control (Management of soil nutrients)
In addition to the management of soil fertility, especially in terms of the macronutrient content (Ou, 1985), significant control of brown spot has been achieved using silicon fertilizers (Datnoff et al., 1991, 1992; Deren et al., 1994).
Combining cattle manure at 20 t/ha with chemical fertilizer greatly reduced the severity of brown spot disease of rice (Mathew, 1996).
Host-Plant Resistance
Many trials have been conducted over a number of years to select for host-plant resistance to C. miyabeanus (Ou, 1985). This appears to have been undertaken without a clear understanding of the variability which may exist in the pathogen (Misra, 1985; Malavolta et al., 1992), and this may have considerably hampered progress in this field. There is also controversy as to the nature and inheritance of resistance; Nagai and Hara (1930) reported that resistance is dominant, while Adair (1941) reported that it is recessive and involves several genes.
In the USA, Lafitte (PI593690) is an early maturing, high-yielding, medium grain cultivar noted for its resistance to three pathogens including C. miyabeanus (Linscombe et al., 1997). Satyam and Kishori, two new rice varieties released for general cultivation in the rainfed lowlands of Bihar, India, are tolerant to brown spot (Thakur et al., 1998). The glutinous rice cultivar Bora 15 was highly resistant to and cultivars Bhogali and Rongili were tolerant of brown spot in Assam, India (Rathaiah, 1997).
Chemical Control
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any
specific chemical control recommendations. For further information, we recommend you visit the following resources:
Brown spot is a very common disease of rice worldwide. It can cause considerable yield losses; the disease was seen as the main cause of the Great Bengal Famine of 1943, which resulted from yield losses ranging from 40 to 90 % in the previous year (Padmanabhan, 1973). The effects of C. miyabeanus on yield should be determined both quantitatively and qualitatively (Ou, 1985). However, the injury-damage relationships are poorly documented. The following are some examples of the effects of C. miyabeanus on rice: 10-58 % seedling mortality in the Phillipines (Ocfemia, 1924); reduced tiller number, 'severe' disease intensity and 20-40% yield reduction in India (Vidhyasekaran and Ramadoss, 1973); 'severe' disease intensity and 30-43% yield reduction in Nigeria (Aluko, 1975); early senescence and yield reduction (33%) in Suriname (Klomp, 1977); and reduced seed germination (40%) and seedling height (3-20%) in Sumatra (Zulkifli et al., 1991).
