Greenhouse Disease Management
Phytophthora Root Rot
Phytophthora is a water mold (like Pythium) and can be a particularly devastating and difficult-to-control problem. Phytophthora is a serious disease of floriculture crops, and it is especially difficult to control once it has become established. It can spread quickly in a greenhouse, especially in flood floor and hydroponic systems. It is one of several organisms that cause root and crown rots. Phytophthora is closely related to Pythium, which also causes root rot. While there are a wide variety of plants that are susceptible to Phytophthora, some are more likely to be infected than others. Poinsettia, fuchsia, vinca, gloxinia, lantana, African violet, begonia, and ornamental peppers are among the more common hosts of the disease. There are several species of Phytophthora, and they are host-specific, meaning each species will only affect certain types of plants.
Disease Cycle
Prevention of Phytophthora is especially important, because it is difficult to eradicate once infection begins. It produces several different types of spores that allow it to persist through a variety of environmental conditions. Oospores and chlamydospores can survive on pots, floors, media and other surfaces, while zoospores can actually swim for tiny distances (a few inches) on a film of water.
Symptoms
Infected plants will display symptoms of wilting and overall decline, including dark lesions, known as cankers, at the crown of the plant (See Figure 28.6). The cankers impede the flow of water and nutrients between the roots and leaves. When they become large enough to encircle the stem, the plant dies. In some cases, the crowns will be the first plant part to become infected, after which the infection will move up the stem into the foliage near the petiole.
Cultural Management Strategies
Controlling the spread of Phytophthora spp. can be difficult. First, Phytophthora must be kept out of the greenhouse. This is particularly difficult with floriculture crops because of the widespread distribution of prefinished plants. Also, plants may not exhibit obvious symptoms until the infection is well established or the plant becomes stressed (e.g., over- or underwatered). Infected plants treated with fungicides may appear healthy until the fungicides wear off and Phytophthora increases.
Chemical Management Strategies
If fungicides, such as the standard Subdue MAXX (mefenoxam), are used to control Phytophthora, rotate it with other effective products. Adorn (fluopicolide), Micora SC (mandipropamid), Orvego SC (ametoctradin/dimethomorph), and Stature (dimethomorph) have been effective in controlling various Phytophthora species in greenhouse studies. Phosphorous acid products, such as Alude and Vital, may limit disease, however, due to their lack consistency, they should not be relied upon for Phytophthora control
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