Occurrence and biology of Phytophthora parasitica and other plant pathogenic fungi in irrigation water.

Abstract

Phytophthora parasitica, P. citrophthora, and other plant pathogenic fungi were isolated from re-cycled water used to irrigate citrus and other crops. The several propogules of P. parasitica were then studied to determine their survival capabilities in soil and irrigation water. Chlamydospores of P. parasitica were present in field soils from foot-rot infested citrus groves and persisted for at least 60 days in air-dried or moist soils. They germinated in irrigation water or moist soil and formed sporangia within 16 hr. Sporangia were also present in these field soils and survived for at least 60 days in moist soil. They germinated, releasing zoospores into irrigation water 5 min after being inundated. Zoospores were not present in water flooded on air-dried field soil until after 20 hr incubation. Citrus leaves became infected by zoospores within 15 min when placed in zoospore infested water. Although they remained motile in irrigation water for up to 20 hr at 20 C, zoospores encysted when agitated or upon the addition of nutrients, orange peel, or citrus leaves. At low nutrient levels (< 5 mg glucose/liter of sterile distilled water) zoospores germinated and upon cessation of growth the protoplasm contracted within the hyphae and pseudo-septa were formed. Empty cysts or hyphae often lysed; remaining hyphal fragments containing protoplasm survived for at least 40 days at 25 C in untreated waste water and resumed growth upon addition of nutrients. At higher nutrient levels (10-1,000 mg glucose/liter of sterile distilled water) the hyphal tips often produced appressorium-like structures when in contact with the container surface. Exudates from orange peel or citrus leaves stimulated similar activity. The appressorium-like structures usually germinated to produce microsporangia when the nutrients were replaced with untreated irrigation waste water. Some microsporangia persisted in untreated waste water at 25 C for 60 days but most germinated sooner, producing only a single zoospore. Mycelial inoculum from these zoospores was pathogenic to roots of citrus seedlings. Results indicate that P. parasitica is spread by re-cycled irrigation water and that zoospores, or structures produced by them, can play a significant role as survival or dispersal units in re-cycled water.