Phytophthora Root Rot of Alfalfa

Phytophthora Root Rot of Alfalfa


This disease is caused by Phytophthora megasperma Drechs. F. sp. medicaginis T. Kuan ; D.C. Erwin. Hosts of the organism include alfalfa, black medic and birdsfoot trefoil (corn?). The disease is most common when and where free water is available in the soil, either by excessive rainfall or irrigation. Field's with a high clay content are also more likely to have root rot. The optimum soil temperatures for infection range from 24-27oC. Most isolates of the fungus are active in this temperature range but a high temperature isolate (HTI) has also been identified. It has an optimum temperature range of 27-33oC and a maximum of 39oC. Infections resulting from the high temperature isolate are not as severe as those from the moderate temperature isolate. The fungus over seasons as oospores or mycelium in plant debris and as oospores free in the soil. The latter have the greatest capacity for long term survival. There is also some evidence that the fungus can survive between seasons as chlamydospores. The fungus is spread via oospore contaminated soil attached to machinery and on hooves of animals and by zoospores in water. The disease cycle begins when oospores germinate and produce sporangia which release zoospores. Sporangia may also germinate and produce mycelium or other sporangia. Zoospores are disseminated by free water which may result from flooding or irrigation. They may also be spread via runoff water that is collected and reused on fields as a means of irrigation. The zoospores are attracted to the roots by plant nutrients released into the soil (chemotactic attraction). The most common sites of infection are tips of small roots and the base of small lateral roots.<

Symptoms


Plants may be infected at any stage of development. When seedling stands are infected damping off may occur. Above ground symptoms on older plants consist of wilting and a yellow to reddish discoloration on the lower leaves. Plants appear stunted and do not exhibit good regrowth after cutting. Below ground both rootlets and the taproot show varying degrees of rot. Rotting of the taproot may occur at any depth. Affected taproots are initially yellow then become black. There is a sharp line between affected and healthy tissue. In severe cases the entire tap root rots and the plant dies. If the soil dries out and rises in temperature the disease is often arrested. At this time, new side roots may develop from "healthy" areas on the taproot. Branching above the rotted off area is common and often results in a forked appearance.

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Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

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This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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Copyright (C) 2008 J.E. Partridge, University of Nebraska-Lincoln. All Rights Reserved.