Issue 10, June 25, 2010

Pythium Root Rot of Garden Plants

Root rot problems are often difficult to diagnose and can be confused with environmental stress. Above-ground root rot symptoms might include stunted growth, smaller than normal leaves, poor foliar color, dieback of stems, or sudden wilt and death of plants. Additionally, any factor that limits root growth might produce nutrient stress symptoms. Dig up an affected plant, dip it in a bucket of water several times to gently wash soil off the roots, and then assess root health.

Plants with Pythium root rot will have blackened root tips or soft, dark rot of the outer (cortex layer) of the roots. Pythium typically leaves the center of the root (stele) white and firm. A diagnostic tip for Pythium root rot involves holding the infected root and gently pulling. The outer, rotted tissue easily slides off the stele. The image shows chrysanthemum roots with Pythium root rot. Phytophthora root rot tends to turn the entire root brown or black, inside and out. Fusarium root rot and Rhizoctonia root rot cause dry rots and turn roots reddish brown.

Environmental factors can cause symptoms similar to those caused by root rots. Excess water, drought conditions, or low nitrogen conditions can produce similar symptoms. The excessively wet conditions of the past weeks can injure roots, causing decline even before root rot pathogens invade. Add to this the fact that Pythium and Phytophthora are water molds, infecting roots in wet conditions.

Pythium infects young feeder roots or root tips first. It then moves into the rest of the root system or basal stems. There are many species of Pythium in the soil, some aggressive root invaders and some infecting only dead plant material (saprophytes). Pythium may survive for several years in soil and plant refuse as thick-walled spores called chlamydospores or oospores.

You won't be able to cure a plant infected with Pythium root rot, but an accurate diagnosis will help you manage the disease and possibly keep it from spreading. Most plant labs can identify this pathogen fairly rapidly. Oospores and sporangia may appear in incubated tissue within 24 hours. The image shows an oospore in infected chrysanthemum roots. Cultures take longer to develop but provide more evidence of the pathogen's presence.

Chemical options are used to protect nearby plants. Chemicals that have efficacy against Pythium include mefenoxam (Fenox, Mefenoxam, Subdue), propamocarb (Banol), etridiazole (Truban, Terrazole, Banrot) and aluminum tris (Aliette, Fosetyl). Some greenhouse growers have found that Pythium can develop resistance to mefenoxam. The same can be true of propamocarb. Fungicide resistance can be avoided by rotating chemical classes, especially in greenhouses where frequent fungicide applications are necessary. Always read the label of any chemical used to be certain it is cleared for use on the targeted host. Most importantly, improve drainage on the site to move water out of the planting bed quickly. Be selective when buying plants and choose only those with healthy looking roots.

Plants that often host Pythium Root Rot include geranium, chrysanthemum, celosia, dianthus, marigold, and others that do not grow well in wet conditions.--Nancy Pataky

Nancy Pataky

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