There are many growers of garden mums in Delaware. This is a good income generating crop for the fall period. Growing good mums requires keeping on top of pests including diseases. The following is information on diseases to watch for in garden mum production.
Common diseases diagnosed in recent years have been root rots caused by Pythium and Rhizoctonia and bacterial leaf spot (Pseudomonas cichorii). Fusarium wilt occurs when cuttings are infected or soil is contaminated with the pathogen. Leaf spots caused by Alternaria, Botrytis and Septoria are not common on mums but occur ocassionally. White rust (Puccinia horiana), also not common, has shown up in the Northeast in recent years.
Mums suffering from root diseases will often look nutrient deficient and they are, considering the roots are not functioning correctly. Plants with root rot often look stunted, wilted and have veinal reddening even when adequate nutrition is being applied. Fungicide drenches for root rots include, thiophanate methyl & etridiazole (Banrot) for Pythium and Rhizoctonia; mefenoxam (Subdue MAXX), etridiazole (Truban) propamocarb (Banol) for Pythium; and fludioxonil (Medallion), thiophanate methyl (Cleary's 3336, Fungo Flo) or iprodione (Sextant) for Rhizoctonia.
Fusarium is a vascular disease that develops within stems. Fusarium wilt results in symptoms similar to root rot but plants infected with Fusarium generally wilt in sectors, or one branch at a time and roots often appear healthy. Root rot usually results in the entire plant wilting. In later stages of Fusarium wilt, a white or pinkish fluffy mold may develop on the affected stems. Managing Fusarium centers on disease-free cuttings and pathogen-free root media. Thiophanate methyl (Cleary's 3336) and fludioxonil (Medallion) have been reported to suppress Fusarium.
Bacterial leaf spot, caused by Pseudomonas cichorii often occurs during hot humid weather in August. This disease tends to be problematic during years of heavy rains or where overhead watering is practiced. Plants with this disease have large black spots concentrated at the base of the plant. The spots often begin at the leaf margin but may also occur randomly. From the leaf, the bacterium can move through the petiole and into the stem resulting in a canker. The sepals of infected flower buds will become brown to black and up to several inches of pedicel may be killed. Copper hydroxide sprays such as Kocide 101 77 WP or Phyton 27 will help protect against this disease. These materials do not cure the disease, they limit spreading to uninfected plants. Also there are differences in cultivar susceptibility. Make notes when you see susceptible varieties and avoid growing them in the future.
Alternaria, Botrytis and Septoria can be managed using thiophanate methyl (Cleary's 3336), iprodione (Sextant), chlorothalonil (Daconil Ultrex, Pathguard 6F), chlorothalonil & thiophanate methyl (Spectro 90 WDG) or fludioxonil (Medallion). As the plants grow rapidly in August and develop a dense canopy of leaves, treatments may be necessary. White rust (Puccinia horiana) is a federally quarantined pathogen. Look for white to yellow spotting on the upper sides of the foliage with corresponding pustules of white spores which look bumpy on the undersides of leaves. Cooler weather conditions in fall tend to favor this disease. Although not a common disease, be aware of it. Materials used to protect against rust contain chlorothalonil (Daconil Ultrex, Pathguard 6F), mancozeb (Dithane T/O), azoxystrobin (Heritage), trifloystrobin (Compass), propiconazole (Banner Maxx) and triadimefon (Bayleton). Bacterial leaf spot and foliar diseases are spread by splashing water, which is why we see more of these diseases during rainy years. Drip irrigation helps to prevent foliar diseases. If overhead watering, foliage should always be dry before evening hours.
Reprinted from "Garden Mums - Past Crop Problems and Production Tips" by Tina M. Smith, Extension Floriculture Program, UMASS.
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