RESUMO
Phakopsora meibomiae (Arthur) Arthur has been reported to occur in several legume species in the tropical regions of Central and South America. In Costa Rica, this pathogen was initially reported as P. pachyrhizi Sydow (1); however, to our knowledge, P. pachyrhizi has not been detected in Costa Rica. In routine evaluations of a 0.2-ha field planted with soybean (Glycine max (L.) Merr var. CIGRAS 06) in La Garita, Alajuela, Costa Rica, symptoms similar to Asian soybean rust were observed in December 2012 and January 2013. Soybean plants were at growth stages R4 to R5 when these symptoms were observed, which included yellow spots on leaves with brown spots on the abaxial surface. Further evaluations at growth stage R5 to R6 indicated that the spots had coalesced, turned grayish-brown, and caused substantial defoliation. Microscopic examination of symptomatic leaves showed the presence of uredinia and urediniospores on the lower surface of the leaf. While initial symptoms were on the southern side of the field, a substantial area of the field was infected at the second evaluation. Infected leaves were submitted to the USDA-ARS Foreign Disease-Weed Science Research Unit under the appropriate USDA-Animal Plant Health Inspection Service permit for molecular characterization and identification. Urediniospores were collected by washing infected leaves with sterile water and then pelleted by centrifugation. DNA was extracted from urediniospore pellets and excised leaf pieces using a DNeasy Plant Mini Kit (Qiagen, Germantown, MD), and eight samples were amplified in real-time polymerase chain reaction (PCR) with P. pachyrhizi-specific primers Ppm1 and Ppa2 but not with the P. meibomiae-specific primers Ppm1 and Pme2 (2). Nucleotide sequence alignment of the internal transcribed spacer (ITS) regions 1 and 2 that were amplified by PCR using the primers Ppa1 and Ppa2 further confirmed the identification as P. pachyrhizi. To the best of our knowledge, this is the first known confirmation of soybean rust, caused by P. pachyrhizi in Costa Rica. CIGRAS-06 is the only soybean variety bred in the country as well as one of the very few varieties available for growers. Given that breeding for disease resistance is not a short-term option for P. pachyrhizi, alternative disease management strategies will have to be developed. References: (1) K. R. Bromfield. Soybean Rust, Monograph No. 11. APS Press, St. Paul, MN, 1984. (2) R. D. Frederick et al. Phytopathology 92:217, 2002.
RESUMO
Fusarium verticillioides causes seedling decay, stalk rot, ear rot, and mycotoxin contamination (primarily fumonisins) in maize. Systemic infection of maize plants by F. verticillioides can lead to kernel infection, but the frequency of this phenomenon has varied widely among experiments. Variation in the incidence of systemic infection has been attributed to environmental factors. In order to better understand the influence of environment, we investigated the effect of temperature on systemic development of F. verticillioides during vegetative and reproductive stages of plant development. Maize seeds were inoculated with a green fluorescent protein-expressing strain of F. verticillioides, and grown in growth chambers under three different temperature regimes. In the vegetative-stage and reproductive-stage experiments, plants were evaluated at tasseling (VT stage), and at physiological maturity (R6 stage), respectively. Independently of the temperature treatment, F. verticillioides was reisolated from nearly 100% of belowground plant tissues. Frequency of reisolation of the inoculated strain declined acropetally in aboveground internodes at all temperature regimes. At VT, the high-temperature treatment had the highest systemic development of F. verticillioides in aboveground tissues. At R6, incidence of systemic infection was greater at both the high- and low-temperature regimes than at the average-temperature regime. F. verticillioides was isolated from higher internodes in plants at R6, compared to stage VT. The seed-inoculated strain was recovered from kernels of mature plants, although incidence of kernel infection did not differ significantly among treatments. During the vegetative growth stages, temperature had a significant effect on systemic development of F. verticillioides in stalks. At R6, the fungus reached higher internodes in the high-temperature treatment, but temperature did not have an effect on the incidence of kernels (either symptomatic or asymptomatic) or ear peduncles infected with the inoculated strain. These results support the role of high temperatures in promoting systemic infection of maize by F. verticillioides, but plant-to-seed transmission may be limited by other environmental factors that interact with temperature during the reproductive stages.