In a coffee (Coffea arabica) farm located in Ervália, Minas Gerais State, Brazil, we observed castor bean (CB; Ricinnus communis) plants growing as weeds. Currently, there is increasing interest in CB as a crop in Brazil because it is an alternative source for biofuel production, but there is little knowledge on CB diseases. According to reports from Costa Rica (2) and Malawi (3), Cercospora coffeicola can cause leaf spots in CB plants. Considering the increasing importance of Cercospora leaf spot on coffee and the expansion of CB plantings in Brazil, in 2008 we evaluated whether CB plants were susceptible to C. coffeicola. Healthy seedlings of an unknown CB cultivar were collected from the experimental coffee area in Ervália, transplanted to pots, and kept under greenhouse conditions. Each of two C. coffeicola isolates from coffee, obtained from the collection of the Departamento de Fitopatologia, was inoculated on two CB plants and three ‘Catuaí Vermelho’ coffee plants, each with four fully expanded leaves. Conidia were produced following a standard protocol (4). A suspension with 1 × 104 conidia ml–1 was sprayed with a DeVilbiss atomizer on both leaf surfaces until runoff, dispensing approximately 3 ml per leaf. As controls, two CB and three coffee plants were sprayed with distilled water. All treated plants were kept in a dew chamber at >90% relative humidity, 25°C, and with 12 h per day of light. After 48 h, the plants were placed in a greenhouse with natural lighting and an average temperature of 25 ± 3°C. Plants were checked at 3-day intervals for disease symptoms. On average, the incubation period (time between inoculation and appearance of the first leaf symptom) was 9 days and the latent period (time between inoculation and visualization of the first leaf sporulating lesion) was 12 days for the disease in CB. Both C. coffeicola isolates were pathogenic to all inoculated CB plants, in which approximately 20 spots developed per leaf. Control plants did not develop any symptoms. Leaf spots in CB plants were similar to those previously described (2): necrotic lesions that were black and purple with a yellowish halo and a pale white center. There was intense sporulation at the lesion center, and the conidia were hyaline, acicular to obclavate, nearly straight with truncate to subtruncate bases and acute tips, multiseptate, 2 to 4 × 40 to 150 μm and were produced in fascicles of conidiophores that were pale to medium brown, septate, and 4 to 6 × 20 to 275 μm (1,4). The pathogen was reisolated from the CB leaf spots, grown on potato dextrose agar medium, and the morphology of the colonies was similar to the colonies of the isolates from the C. coffeicola collection. The reisolated cultures were set to sporulate and reinoculated onto new healthy coffee and CB leaves, on which leaf spots developed. To our knowledge, this is the first report of castor beans as a host of C. coffeicola in Brazil. Considering the expansion of CB crops in Brazil, studies are needed to evaluate the susceptibility of the commercial CB cultivars that are grown in the country, particularly those planted close to coffee-production areas where Cercospora leaf spot is endemic and important.
References: (1) C. Chupp. A Monograph of the Fungus Genus Cercospora. Charles Chupp, Ithaca, NY, 1954. (2) E. Echandi. Turrialba 9:54, 1959. (3) M. A. Siddiqi. Trans. Br. Mycol. Soc. 54:415, 1970. (4) A. G. C. Souza et al. J. Phytopathol. 159:6, 2011.
