Seedling diseases - alfalfa
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Strong seedling establishment in alfalfa is important to achieve the plant density needed to outcompete weeds and produce high biomass yields. However, establishing alfalfa can be challenging because alfalfa seeds and seedlings are vulnerable to several pathogens present in soil.
Wet soil conditions favor the development of diseases caused by water molds, namely Phytophthora root rot (PRR), Aphanomyces root rot (ARR) and damping off (rotting). The pathogens causing these diseases produce mobile swimming spores called zoospores that require water to develop and infect alfalfa.
When rain is excessive after sowing, stand establishment may fail due to seed and seedling rot caused by these pathogens. Both ARR and PRR can also attack adult plants under wet soil conditions.
Yellow alfalfa and Aphanomyces root rot
Seedlings infected by ARR become stunted and chlorotic (yellow) before they wilt and die. Infected seedlings usually remain upright.
In adult plants, the root mass is reduced and lateral roots have brown decay. A brown lesion on the taproot may mark the location where lateral roots were rotted off. Nodules are frequently absent or decaying.
Foliage is stunted, becomes chlorotic and resembles symptoms of nitrogen deficiency (Figure 1). Infected plants are often slow to regrow or may fail to grow after harvest or winter dormancy.
The roots of infected plants look like the root of a carrot with mostly just a taproot (Figures 2 and 3). The lack of nodules and fibrous roots is the main cause of nitrogen deficiency symptoms.
Chlorotic foliar symptoms occur in soils that are deficient in sulfur. So it’s prudent to examine the roots of plants for evidence of root rot as well as to test plant tissue or soil for sulfur content to determine the cause of the symptoms.
Minnesota alfalfa fields that have had high rainfall levels may have yellow, stunted or slow-growing plants.
The problem is more pronounced in heavier, poorly drained soils. The problem likely results from a:
Lack of oxygen to plant roots due to the saturated soil.
Possible infection by the water mold Aphanomyces euteiches, which causes Aphanomyces root rot of alfalfa.
This pathogen has swimming spores that are active in wet soil. Once the pathogen infects the roots, it degrades the fine fibrous roots where root nodules are located and degrades the lateral roots (Figures 2 and 3).
Once soils dry out, plants may recover if there’s sufficient time before frost occurs. Stress from waterlogging and disease may increase winter injury of these stands, leading to greater-than-normal stand losses the next spring.
Two races of Aphanomyces occur in Minnesota, so select cultivars with this problem in mind. However, under continuous heavy rainfall with wet soil conditions, even resistant cultivars will show signs of Aphanomyces root rot.
Phytophthora root rot
Seedlings infected by PRR collapse and rapidly decay. In established stands when soils remain wet, PRR attacks lateral roots and the taproot. The rotted tissue turns dark brown-black, forming a pencil point-like symptom, and foliage turns yellow or reddish.
To help protect seedlings from seed rot, damping-off and PRR, the majority of alfalfa seed is treated with the systemic fungicide mefenoxam, Apron XL. However, Apron XL doesn’t provide protection against ARR.
Varieties with resistance to ARR are available, but the majority of varieties have resistance to only one race (race 1) of the pathogen.
A recent survey of 45 Minnesota alfalfa fields found that a second race (race 2) is widespread in the state and more common than race 1. The same situation has also been found in Wisconsin and New York. Evidence is mounting that additional races of ARR are present in alfalfa fields that can overcome race 1+2 resistance.
Recently, the fungicide Stamina, which protects seedlings against ARR, was labeled for use as an alfalfa seed treatment. It’s being used along with Apron XL to help boost protection from soilborne pathogens.
Reviewed in 2018