台灣地區番椒疫病菌 (Phytophthora capsici) 之生物特性暨抗病品種之篩選

Abstract

Phytophthora blight of pepper (Capsicum spp.) caused by Phytophthora capsici L. is one of the major limiting factor of pepper production worldwide. Several reports had identified the existence of pathogenic specificity of P. capsici on peppers. Pathotypes are determined by the pathogen reaction to a set of differential hosts. The identification of the pathotype of P. capsici has been commonly used the breeding program for disease resistance. Our previous studies indicated that only the A1 mating type isolates of P. capsici was found in Taiwan befoe 2007. The A2 mating type isolates were found on tomato, eggplant and pepper since 2007. Metalaxyl related chemicals have been used in the control of many different oomycete pathogens more than 20 years in Taiwan. The objectives of this study are: i.) To understand the characteristics of P. capsici on pepper in Taiwan including the characterizations of pathotype, mating type and in vitro assessment of metalaxyl sensitivity and ii.) To screen the resistance variety from wild peppers and to evaluate the resistance of AVRDC advanced lines for disease control. Four pepper varieties/lines, Early Calwonder, PBC 137, PBC 602 sel and PI 201234, were used as indicator plants to classify all of P. capsici isolates into three pathotypes, type1, 2 and 3. Pathotype 3 showed the highest virulence and was the predominant population in Taiwan. The growth rate of pathotpe 3 was slowler than pathotype 1 and 2 on both PDA and V8 plates. Pathotype 1 isolates produced small and round sporangia (43±3 × 33±2 μm, L/B: 1.3±0.06). The sporangia of pathotype 2 were 50±6 × 35±5 μm, L/B: 1.4±0.09, while those of pathotype 3 were 49±4 × 33±3 μm, L/B: 1.5±0.15. Pathotype 1 was significantly different from pathotype 2 and 3 based on PCR polymorphisms using primers from P. capsici elicitin and AFLP analysis. The comparison of the difference of morphology, zoospore productivity, growth rate, pathogenicity between A1 and A2 mating type isolates of P. capsici showed that the growth rate of the A2 mating type isolates was slower than A1 mating type in both PDA and V8 plates. The A2 mating type isolates produced larger and oval sporangia (50±3 x 31±2 um; L/B: 1.6), and grew even at 36℃. These isolates caused more severe disease because they belonged to either pathotype 2 or 3. A1 mating type was the predominant type in Taiwan. In vitro assessment of metalaxyl sensitivity of P. capsici isolates indicated that 68.7 % of the isolates were classified as sensitive. The only 3 % isolates collected before 2008 were resistant to metalaxyl, while 43.4 % of those collected in 2008 were resistant to metalaxyl. Most pathotype 1 isolates (66.7 % resistant) were resistant to metalaxyl than pathotype 2 and 3 isolates (7.0 % and 15.1 % respectively). More A2 mating type isolates (13.3% resistant) were sensitive to metalaxyl than A1 isolates (55.3% resistant). Resistance screening of wild germplasms collected from GRSU (Genetic resources and seed unit) of AVRDC and evaluation of resistance in AVRDC advanced lines by root-drench noculation were conducted. One wild accessions of TC05415 in Capsicum frutescens showing resistant percentage higher than 94 % were selected. Six progeny lines (F8) of AVRDC advanced lines exhibited resistant percentage higher than 87.5% and 7 progeny lines of AVRDC advanced lines exhibiting 100 % resistance were selected. The resistant accessions can be used as the resistant parent in the breeding program for disease resistance. Those resistant advanced lines will be evaluated for durable resistance and horticultural traits for the future extension. The impact of the occurrence of the A2 mating type on peppers is not yet clear in Taiwan because it first appeared only in 2008. However, the A2 mating type isolates with tolerance to high temperature and ability to produce more zoospores as well as the genetic variation followed by sexual recombination between A1 and A2 mating types may lead to the difficulties of disease control in the future. Therefore, it’s important to monitor the population shift of P. capsici and develop the resistant varieties of pepper for the effective control of phytophthora blight.