Greenhouse Disease Management
Fungal Leaf Spots
Fungi cause all of these diseases, with symptoms varying greatly. The more common fungi causing leaf spots include Alternaria, Ascochyta, Cercospora, Colletotrichum (anthracnose), Phyllosticta, Gloeosporium, and Septoria, all part of the ascomycetes group of fungi. Pathogens affect most ornamentals, including bedding plants, cut flowers and cut foliage, tropical foliage plants, and woody crops.
Disease Cycle
Many of the leaf spot fungi have a similar life cycle. Most produce abundant conidia. Spores of these fungi are wind-borne and can be splashed around when watering. Some of these leaf spot fungi can be seed-borne.
Symptoms
Blights or spots can appear on leaves, stems or flowers depending on the organism present (See Figure 28.4). Many attack all three of the plant parts, but none attack the roots or crown. Leaf blights and spots usually appear on the lower foliage and work their way up through the plant canopy.
Cultural Management Strategies
Prolonged leaf wetness usually favors fungal leaf spot disease development. Inspect plants on a regular basis for fungal leaf spot symptoms. Discard symptomatic plants. Reduce humidity in the greenhouse.
Chemical Management Strategies
There are many broad-spectrum fungicides with wide crop clearance which will control most, if not all, leaf spots without an accurate diagnosis including axoystrobin, chlorothalonil, mancozeb, mylobutantil, pyrclostrobin, thiophanate-methyl, and copper products. Alternaria leaf spot can be controlled with many different fungicides. Some of the oldest fungicides, including chlorothalonil and mancozeb, can give excellent control of Alternaria, but both leave residue that’s unacceptable sometimes. Anthracnose caused by Colletotrichum spp. can be controlled effectively with copper, mancozeb, chlorothalonil, and Pageant Intrinsic (combination of FRAC groups 7 and 11). Using a rotation of copper sulfate pentahydrate with Pageant should give good control of anthracnose diseases without excessive residue. Appendix C, Fungicides and Bactericides Labeled for Greenhouse Use, lists fungicides and bactericides, labeled for control of plant diseases in greenhouses.
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