Phytophthora blight, caused by Phytophthora capsici, constitutes a limiting factor to profitable production of many crops worldwide (Erwin and Ribeiro, 1996), including chile pepper (Sanogo, 2003, 2004; Sanogo and Carpenter, 2006). Phytophthora blight was first described by Leonian (1922) as a disease affecting roots, stems, leaves, and fruit of chile pepper. Within 5 d of inoculation, P. capsici can cause death of chile pepper plants, especially when plants are young. Infection of the plant stem is indicated by dark purplish lesions close to the soil surface. As the disease progresses, lesions rapidly coalesce and completely girdle the stem. Chile pepper fruit become infected during prolonged periods of heavy rain and high humidity in flooded and poorly drained fields. Infected fruit initially show small, water-soaked, dull green lesions that rapidly shrivel and rot (Biles et al., 1992). Severely diseased fruit are covered with a white powdery mat as a result of mycelium and sporangia production on the infected fruit surfaces.
The most effective means for controlling Phytophthora blight are chile pepper cultivars that are resistant to P. capsici. Although genetic resistance has been identified in some chile pepper lines (Bosland and Lindsey, 1991), currently there are no commercial chile pepper cultivars that are resistant to P. capsici in all environments. At present, management of Phytophthora blight relies on reducing saturation conditions in field soil to minimize spread and reproduction of P. capsici.
Chile peppers (nonbell-type) are characterized by the presence of capsaicinoids in fruit, and capsaicin and dihydrocapsaicin account for more than 80% of the total capsaicinoids produced (Zewdie and Bosland, 2001). Capsaicin and related alkaloids evoke pain equivalent to pain evoked by heat on noniceptors, which are pain-sensing neurons (Caterina et al., 2000). Therefore, capsaicin-evoked pain is best described in terms of burning or heat sensation. In this study, the term “heat” is used in lieu of “pungency”, which is a term commonly used in the literature on “hot” peppers. Heat level in chile pepper is expressed in Scoville heat units (Scoville, 1912). According to Bosland and Votava (2000), the following ranges of Scoville heat units (SHU) are used for classifying chile pepper products: 0 to 700 SHU (low heat), 700 to 3,000 SHU (mild heat), 3,000 to 25,000 SHU (moderate heat), 25,000 to 70,000 SHU (high heat), and greater than 70,000 SHU (very high heat).
Chile pepper producers in New Mexico and Arizona observed that Phytophthora wilt symptoms develop slower and its incidence is lower in hot than in low-heat chile pepper cultivars. On a farm in South Carolina, Keinath (2007) observed that disease incidence was higher on bell pepper than on the jalapeño type. Although no explanation for this observation was provided, it is possible that this differential incidence between the two pepper types is because jalapeño is a hot chile pepper type in contrast to bell pepper.
Information on the relationship of chile pepper heat level to diseases and plant pathogen populations is scanty, observational, and inconsistent across pathosystems. Blazquez (1976) reported that powdery mildew, caused by Oidiopsis sp., was less severe on most no-heat cultivars than on jalapeños. It has been observed that production of microsclerotia of Verticillium dahliae was less in jalapeño fields (Bhat et al., 2003) than in nonjalapeño fields, which may be attributed to the fact that jalapeño is a hot chile pepper.
There has been no previous systematic assessment of the relationship of chile pepper heat level to chile pepper response to P. capsici. Elucidation of this relationship may reveal features usable in breeding programs intended for developing resistance to P. capsici. In laboratory experiments, Beard (2006) showed that mycelial growth of P. capsici was reduced by 49% to 83% on media amended with Capsicum oleoresin varying in heat level from 100,000 to 300,000 SHU relative to nonamended media. These results suggest that infection and colonization of hot chile pepper tissue by P. capsici may be reduced compared with low-heat chile pepper tissue.
The hypothesis of this study was that the severity of Phytophthora blight would be greater in low-heat than in hot chile peppers. The objectives of this study were to evaluate the effect of hot chile pepper cultivars on root and fruit infection by P. capsici and the development of Phytophthora blight.
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