Using Temperature Information to Aid in Controlling Diseases of Ornamentals

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University of Florida, IFAS
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-91-8

A. R. Chase and C. A. Conover*

Most plant diseases exhibit a characteristic range of temperatures for optimal development. Often this range is the same as that for optimal production of the host plant, but in other cases, plants can be produced with a temperature range which is not favorable for a particular plant pathogen. For example, Myrothecium leaf spot of foliage plants is most severe when temperatures are in the range of 70 to 81°F (Table 1). Since many foliage plants can be produced at temperatures above 81°F, Myrothecium leaf spot can be avoided during times when temperatures regularly exceed 81°F. In contrast, Rhizoctonia aerial blight can develop over a relatively wide range of temperature from as low as 59°F, but cease development if the potting medium temperature exceeds 90°F or the air temperature exceeds 95°F. Such conditions are not unusual during summer months in Florida and Texas but would not be common in many other foliage growing areas such as Hawaii and California. The relatively narrow range of optimum temperatures for development of Botrytis blight (64 - 72°F) explains why this disease is not as common in subtropical climates as in temperate climates even when susceptible plants are produced.

Knowing the optimal temperatures for development of a given disease is best used for timing control measures rather than altering the growing environment. This information is also helpful to disease diagnosticians since they can make special effort to look for target pathogens at certain times of the year. Hibiscus produced in Florida are subject to three bacterial leaf spot diseases which have different temperature optima. During winter months, Pseudomonas syringae pv. hibisci occurs with Pseudomonas cichorii common during the spring and fall. In general, only Xanthomonas campestris pv. malvacearum is active on this plant during summer months. Although certain diseases tend to occur during the winter and others only during the summer, growers should be prepared for exceptions. Production innovations such as under bench heating have affected more than plant production and fuel costs during winter months. Diseases such as Cylindrocladium root rot of Spathiphyllum spp. which usually do not develop during the cooler winter months, now occur year round when plants are produced with under bench heating.

Scouting for diseases only at times when temperatures are favorable allows for better management of personnel resources. In addition, when preventive pesticides are recommended they can be applied only when disease development is possible and not on a year round basis, which is costly and potentially hazardous to plants and workers, and could also lead to development of resistant populations.

Tables 1 and 2 give the optimum air temperature ranges for some ornamental diseases. This information was obtained from a variety of references as well as new research. Table 1 can best be used if the disease in question has been correctly diagnosed. If the disease is not known, Table 2 may be more useful since it is arranged by plant rather than by pathogen. Although these tables represent the majority of the information available on temperature ranges of ornamental diseases they fall short of covering all ornamental diseases. If a specific disease is not listed for the plant you are interested in checking, it is likely that the same disease will act similarly on another host. This can readily be seen by examining the information on Rhizoctonia diseases which are all most severe with temperatures between about 70 and 90°F. Be sure to obtain a reliable diagnosis whenever you encounter a new problem or one that seems to occur at an unusual time of year or under suboptimal temperature conditions. Many diseases produce similar symptoms and laboratory culturing is necessary for an accurate diagnosis.

*Professor of Plant Pathology, Center Director and Professor of Environmental Horticulture (retired 7/96), respectively. Central Florida Research and Education Center - Apopka, 2807 Binion Rd., Apopka, FL 32703-8504.