Greenhouse Disease Management
Black Rot
Black root rot, caused by the fungus Thielaviopsis basicola, is a serious threat to pansies, petunias, and vinca. It may also infect cyclamen, calibrachoa, poinsettia, primula, impatiens, snapdragon, verbena, phlox, begonia, and nicotiana. Symptoms of black root rot are often mistaken for nutrient deficiencies. It causes little to no symptoms on some cultivars and species, so it can easily be passed from greenhouse to greenhouse on healthy-appearing plants. On highly susceptible cultivars; however, Thielaviopsis causes serious growth reduction and crop losses that are quite dramatic (e.g., pansy and vinca). Some crops, like poinsettias, can go for years without any outbreaks of black root rot. Once the disease becomes established in an operation it can be very tough to eradicate.
Disease Cycle
The fungus is soil-borne and is capable of living in soils as a saprophyte (without causing disease) and surviving in soil and dust for years as dark colored chlamydospores. These dark colored spores give an infected plant parts a black coloration which has resulted in the common name, black rot.
Symptoms
The black root rot fungus damages the root of the plant, effectively interfering with the root's ability to absorb nutrients (See Figure 28.1). As a result of root injury, plants usually develop symptoms indicative of nutritional stress. Plants with black root rot often show symptoms that mimic nutrient deficiencies such as stunting with older leaves shriveling. Leaves may turn yellow and the youngest leaves become stunted and tinged with red. In mild infections, older leaves are yellow-green with the veins retaining their green color.
Cultural Management Strategies
Sanitation is the best preventive measure against black root rot. This pathogen produces a spore that can persist on floor mats, greenhouse benches, or flats/pots. It is not recommended to reuse plug trays for crops that are susceptible to Thielaviopsis. If you are considering reusing pots or plug trays, select those crops less susceptible to Thielaviopis for replanting in reused containers. Powerful rinsing is needed to remove organic debris that can harbor Thielaviopsis. Use a disinfectant to thoroughly clean the pots and plug trays. The disinfectants hydrogen dioxide or a 10 percent solution of chlorine bleach is most effective as disinfectants for pots and plug trays. At the end of the growing season, do a thorough clean up of the greenhouse.
Chemical Management Strategies
Plants cannot be treated with a fungicide once infected with the fungus Thielaviopsi. Fungicides applied as a soil drench can help protect roots from Thielaviopsis infection. The best and most reliable fungicides for black root rot have included the active ingredient thiophanate-methyl. While other fungicides have sometimes given some control, those containing a benzimidazole have always provided the best control. Products that have shown good efficacy at times (but not as consistently as thiophanate-methyl) include azoxystrobin, fludioxonil and triflumizole as active ingredients. Polyoxin D is reportedly very effective in controlling black root but not as effective as thiophanate-methyl.
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