Influence of pH and Etridiazole on Pythium Species

Abstract

Pythium root rot (Pythium sp.) is ubiquitous in Michigan greenhouses that produce herbaceous ornamentals, an industry worth $393 million in the state. Disease symptoms include stunting, flowering delay, root rot, and death. Fungicides that are highly effective against pythium root rot are limited, and pathogen resistance has been documented. The objectives of this study were to determine the sensitivity of Pythium irregulare, Pythium ultimum, and Pythium aphanidermatum isolates from symptomatic herbaceous greenhouse ornamentals to the fungicide etridiazole and to determine the influence of pH and etridiazole on Pythium mycelial growth and asexual reproduction. Isolates were tested in vitro for sensitivity to etridiazole by growing the pathogen on amended V8-agar plates sealed in plastic containers to minimize fungicide loss from the vapor phase. The majority of isolates of all three species were sensitive to the fungicide with EC50 (effective concentration resulting in 50% inhibition of linear growth) values ranging from 0.10 to 5.03 μg·mL−1. Two isolates of P. irregulare had an EC90 (effective concentration resulting in 90% inhibition of linear growth) value >80 μg·mL−1. The acidity of the medium influenced the ability of etridiazole to inhibit Pythium mycelial growth and asexual reproduction. Agar plates amended with 1 μg·mL−1 etridiazole and adjusted to pH 4.5 limited the mycelial growth of two P. aphanidermatum isolates and two P. irregulare isolates by 90% and 56%, respectively, compared with amended agar at pH 6.5. Sporangial formation by P. aphanidermatum was less frequent on mycelial disks incubated in etridiazole-amended sterile distilled water (SDW) at pH 4.5 than pH 6.5 (P < 0.05). P. aphanidermatum zoospore cyst germination was less sensitive to etridiazole than sporangia or mycelial growth; however, the influence of pH and fungicide on cyst germination was significant (P < 0.01). At 250 μg·mL−1 etridiazole and solution pH 4.5, zoospore cyst germination was inhibited 99.9% compared with 94.2% at pH 6.5. In a greenhouse experiment, disease symptoms were observed on ‘Pinto White’ geranium (Pelargonium ×hortorum) in a potting medium infested with P. aphanidermatum and adjusted to pH 4.5 or 6.5; however, plant health and fresh weight were greater in low pH potting medium. Etridiazole, applied as a drench at transplant, did not improve control of root rot for plants grown at low pH (P > 0.05). Fresh weight of plants grown in infested potting medium adjusted to pH 4.5 and amended with a single drench of etridiazole (100 μg·mL−1) was reduced 20%, statistically similar to the untreated control. Adjusting the acidity of irrigation water at the time of etridiazole application in ebb and flow and flood floor production systems could be beneficial in pythium root rot management of certain ornamental crops if plants have tolerance to low pH.