The Infection of Soybean Leaves by Phakopsora pachyrhizi during Conditions of Discontinuous Wetness

The ability of Phakopsora pachyrhizi to cause infection under conditions of discontinuous wetness was investigated. In in vitro experiments, droplets of a uredospore suspension were deposited onto the surface of polystyrene. After an initial wetting period of either 1, 2 or 4 h, the drops were dried for different time intervals and then the wetness was restored for 11, 10 or 8 h. Germination and appressorium formation were evaluated. In in vivo experiments, soybean plants were inoculated with a uredospore suspension. Leaf wetness was interrupted for 1, 3 or 6 h after initial wetting periods of 1, 2 or 4 h. Then, the wetting was re-established for 11, 10 or 8 h, respectively. Rust severity was evaluated 14 days after inoculation. The germination of the spores and the formation of the appressoria on the soybean leaves after different periods of wetness were also quantified in vivo by scanning electron microscopy. P. pachyrhizi showed a high infective capacity during short periods of time. An interruption of wetness after 1 h caused average reductions in germination from 56 to 75% and in appressorium formation from 84 to 96%. Rust severity was lower in all of the in vivo treatments with discontinuous wetness when compared to the control plants. Rust severity was zero when the interruption of wetness occurred 4 h after the initial wetting. Wetting interruptions after 1 and 2 h reduced the average rust severity by 83 and 77%, respectively. The germination of the uredospores on the soybean leaves occurred after 2 h of wetness, with a maximum germination appearing after 4 h of wetness. Wetness interruption affected mainly the spores that had initiated the germination. Introduction Soon after its introduction from Paraguay in 2001, soybean rust (SR) caused severe crop epidemics in Brazil. In 2002 and 2003, the disease had already spread into 60 and 90% of the soybean crop areas, respectively. Losses due to this disease in 2003 exceeded $1 billion and in 2004 over $2 billion (Yorinori et al. 2005). Crop damage in the Eastern hemisphere was also reported at around 10–40% in Thailand, 10–90% in India, 10– 50% in China, 23–90% in Taiwan and 40% in Japan (Sinclair and Hartman 1999). The infection process of Phakopsora pachyrhizi H. & P. Sydow is composed of several distinct stages: uredospore adhesion onto the surface of the host, germination, appressorium formation, direct penetration through the cuticle, invasion and growth of the hyphae into the host tissue (Marchetti et al. 1975; Bonde et al. 1976; Melching et al. 1979; Koch et al. 1983; Koch and Hoppe 1988). Host penetration occurs directly in the lumen of epidermal cells. Less often, the process begins in the space between adjacent cells, and subsequently, the fungus enters into one of these cells (Bonde et al. 1976). Infection success of most pathogenic fungi depends on minimum leaf wetness and varies with the temperature (Huber and Gillespie 1992). Phakopsora pachyrhizi was reported to cause infection after a minimum period of 6 h of leaf wetness at temperatures between 15 and 25 C (Marchetti et al. 1976). Despite the fact that the infection process of P. pachyrhizi has been widely investigated, there is still no information on infection development during discontinuous leaf wetness. Information on the development and biology of P. pachyrhizi infection may aid in developing new control methods and may be useful for the validation of disease simulation models. Our objective was to understand the infection process of P. pachyrhizi and the development of soybean rust under periods of interrupted leaf wetness. J Phytopathol 159:165–170 (2011) doi: 10.1111/j.1439-0434.2010. 01742.x 2010 Blackwell Verlag GmbH