Heat Stress Enhances Phytophthora Root Rot Severity in Container-grown Chrysanthemums

Abstract

Cuttings of Dendranthema ×grandiflorum `Paragon' were used as a model system to assess the effects of root heating on disease severity. Roots were exposed to single episodes of heat stress, after which they were inoculated with zoospores of Phytophthora cryptogea Pethyb. & Laff. Root damage resulting from heat stress, or heat stress plus Phytophthora, was quantified 5 to 7 days after treatment. Roots of hydroponically grown plants, immersed for 30 min in aerated, temperature-controlled nutrient solutions, were severely damaged at 45C or above. Relatively little phytophthora root rot developed on inoculated plants exposed to 25 or 35C, but infection was severe in roots heated to 40C. Plants grown in potting mix were exposed to heat stress by plastic-wrapping the containers in which they were growing and placing them in heated water baths until roots achieved desired temperatures for 30 min. This system heated roots more slowly than in the hydroponic experiments, and 45 and 50C were less damaging. The amount of Phytophthora-induced root damage was insignificant in containerized plants heated to 25 or 35C, but was highly significant in those heated to 40C or higher. In field experiments, plants were positioned so their containers were either fulIy exposed to the late afternoon sun or heavily shaded to prevent sun exposure. The root zones of sun-exposed pots heated to 45 to 47C, while those of shaded pots never exceeded 34 to 36C. There was a large, highly significant increase in phytophthora root rot severity in the sun-exposed pots compared to shaded plants. These experiments showed that temperatures of 40C or higher, which commonly occur in container-grown plants exposed to solar radiation, can predispose chrysanthemum roots to severe Phytophthora infection.