Characterization, population genetics and management of Pythium spp. From floriculture crops in Michigan

Tesis

Michigan ranks third in the U.S. for the wholesale value of floriculture products, with an estimated value of 75.7 million. Seedling damping-off, and root and crown rot are commonly incited by Pythium spp. and are important problems for greenhouse growers. Pythium spp. associated with Michigan's floriculture crops were characterized as a means to improve current management strategies. During 2011 and 2012, symptomatic potted poinsettias were sampled from nine greenhouses in Kent, Kalamazoo and Wayne counties. The following year, from the same three counties, symptomatic geranium and snapdragon bedding plants were sampled from 11 greenhouses. Isolates were confirmed to be Pythium spp. via morphology and sequencing of the ITS region. A total of 1,014 Pythium spp. isolates were obtained; P. irregulare, P. ultimum, and P. aphanidermatum were most prevalent. A subset of isolates was chosen for pathogenicity and mefenoxam sensitivity testing. Most of the Pythium species were virulent to germinating geranium seeds. This study suggests that mefenoxam may not be effective to control P. ultimum or P. cylindrosporum. To understand the population dynamics of the P. ultimum collection simple sequence repeats (SSRs)were developed using P. ultimum transcriptome to address the population structure. After screening in-silico SSR markers, six SSRs were selected based on their polymorphism on a sub set of P. ultimum isolates. A total of 166 P. ultimum isolates were analyzed using the six fluorescent-labeled SSRs. The average genotypic diversity (0.938), evenness (0.56), and the recovery of 12 major clones, out of the 64 multilocus genotypes obtained may suggest that P. ultimum is not a recent introduction into Michigan greenhouses. Analysis revealed a clonal population with limited differentiation among seasons, hosts and counties sampled. To develop strategies that limit Pythium root rot on geranium and snapdragon, greenhouse trials were conducted to test plant protectants and screen cultivars for resistance. Seven fungicides and two biological control agents were evaluated on plants inoculated with P. aphanidermatum, P. irregulare or P. ultimum. The AUDPC values differed significantly (P<0.001)among Pythium spp. and treatments. Mefenoxam and Streptomyces lydicus WYEC108 effectively controlled root rot on geranium. For snapdragon, treatment efficacy varied depending on the Pythium sp. When eleven geranium cultivars, and twelve snapdragon cultivars were inoculated with P. aphanidermatum or P. irregulare, none were completely resistant. However, geranium `Nano White Hybrid' and `Bulls Eye Cherry' and snapdragon `Twinny White' and `Candy Showers Yellow' were least susceptible. Integrating effective fungicides with less susceptible cultivars can limit disease caused by Pythium spp.