First Report of Root and Crown Necrosis of Bean Caused by Pythium aphanidermatum in Spain

Adult bean plants (Phaseolus vulgaris cv. Festival) growing in a commercial greenhouse in southeastern Spain developed symptoms of root necrosis, necrotic streaks on the basal stems, and plant wilt. A Pythium sp. was isolated consistently from roots and basal stems on selective agar (P₅ARP). Single-hyphal transfers produced intercalary antheridia, oogonia (23 to 26 μm in diameter), oospores (18 to 20 μm in diameter), and zoospores in toruloid sporangia. Cardinal temperatures were a minimum of 10°C, an optimum of 28 to 34°C, and a maximum of 40°C. Daily growth rate on corn meal agar at 25°C was 15 mm. The internal transcribed spacer 1 (ITS 1) rDNA sequence of the isolate matched the sequences of Pythium aphanidermatum in GenBank. The sequence of isolate Py-294 was deposited in GenBank, Accession No. AM396563. This isolate was identified as P. aphanidermatum on the basis of morphological and cultural characteristics (1) and the ITS rDNA sequence. To fulfill Koch's postulates, 50 ml of inoculum of isolate Py-294 was used to inoculate bean plants (cvs. Donna and Emerite) at the five-leaf stage. The inoculum was prepared by homogenizing 2-week-old potato dextrose agar-petri plate cultures in 300 ml of distilled water. The plants were maintained in a greenhouse at temperatures of 18.8 to 30.3°C. Irrigation water had an electric conductivity of 0.5 to 0.6 dS·m^-1 while the nutrient solution had 1.9 to 2.1 dS·m^-1. Two months after sowing, 35.4 and 100% of cvs. Donna and Emerite, respectively, developed root necrosis, while control plants grown in bags containing noninoculated perlite remained healthy. The pathogen was reisolated from roots and basal stems of symptomatic plants. The test was repeated with similar results. To our knowledge, this is the first report of P. aphanidermatum as the causal agent of root and crown necrosis of adult bean plants in Spain. Reference: (1) A. J. Van der Plaats-Niterink. Stud. Mycol. 21:242, 1981.