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Identification of Phytophthora cryptogea as the Cause of Rapid Decline of Petunia (Petunia × hybrida) in Chile

Phytophthora cryptogea was consistently isolated from diseased tissue taken from the crown and necrotic roots of grandiflora type petunia (Petunia × hybrida) that were collected in gardens in five public parks in Santiago, Chile in 2004 and 2005. Symptoms included leaf wilting and foliar chlorosis, followed by partial necrosis, and extensive dark-brown to reddish cankers in the crown. Disease incidence was over 50% and infected plants died within 7 to 10 days after transplanting. This pathogen was identified on the basis of colony morphology, morphological characterization of the sexual and asexual reproductive structures, and temperature range. The identification of Phytophthora cryptogea was further corroborated by the internal transcribed spacer sequence analysis (GenBank accession number EF093534). Isolates of P. cryptogea were pathogenic on 10-week-old white grandiflora petunia plants that were inoculated on the roots or on the crown using mycelium fragments, or via soil inoculation using zoospores. A rapid decline was observed after soil inoculations with zoospores. Root fresh weight decreased significantly and the root rot index and severity of foliage symptoms increased significantly (P ≤ 0.05), relative to noninoculated plants after 14 days of incubation. Two isolates (Ph-1 and Ph-2) were pathogenic on bell pepper and one isolate (Ph-1) was pathogenic on tomato after root inoculation. Two isolates (Ph-2 and Ph-3) were pathogenic on the fruit of avocado, bell pepper, cherry tomato, cucumber, kiwifruit, lemon, pear, pepino, and potato tubers, demonstrating the pathogen's ability to cause postharvest infection of fruit of a wide range of host plants. The efficacy of mefenoxam at 0.1 mg/ml mixed with either chlorothalonil at 1.0 mg/ml or mancozeb at 1.6 mg/ml was demonstrated in this study, whereas chlorothalonil and mancozeb alone did not control disease development. No significant differences were obtained between foliage and soil drench applications. This study demonstrated that P. cryptogea is the cause of the rapid decline found on petunia in Santiago, Chile and, to our knowledge, this is the first report giving a detailed description of a disease caused by P. cryptogea on petunia.