Aphanomyces Root Rot In Alfalfa

Aphanomyces Root Rot In Alfalfa

Quality Seeds Introduces Race 2 Resistant Variety “Foundation APR”


Aphanomyces root rot dramatically reduces alfalfa vigour and yields. There are many slowly drained fields in Canada and the US that show symptoms of this soil-borne alfalfa disease. Aphanomyces can be managed by the use of the Race 2 resistant “Foundation APR” alfalfa variety, newly available from Quality Seeds.

Aphanomyces root rot is caused by the fungus-like pathogen Aphanomyces euteiches. Similar to phytophthora root rot (Phytophthora medicaginis), it is considered a major cause of disease in alfalfa seedlings, particularly in wet, saturated, poorly drained soil conditions. Aphanomyces root infection subsequently causes the above-ground symptoms of stunting and yellowing of surviving established alfalfa plants. While phytophthora tends to kill seedlings more quickly and extensively, aphanomyces is considered more chronic. Aphanomyces is less likely to cause seedling death, but more likely to result in stunted, low yielding alfalfa crops.

Wet, Saturated Soils After Establishment

Aphanomyces root rot requires saturated soil conditions for mobility during the initial infection, so poorly drained fields, heavier textured, and compacted soils are higher risk. Aphanomyces symptoms in new seedings are usually not observed in fields where soils are dry after seeding and germination. The aphanomyces organism can survive dormant in the soil for many years, until the saturated soil conditions at the seedling emerged stage are favourable for its growth and development.

Aphanomyces is confirmed in a widespread area in the mid-west and northeast United States, Ontario and other provinces. With delayed germination during a dry May followed by an excessively wet June, many 2015 new alfalfa seedings showed symptoms of aphanomyces that contributed to poor establishment and seeding failures.

What To Look For?

ARR is often challenging to diagnose in the field and currently difficult and expensive to confirm in a lab.

       1. New Seedings

Infected seedlings are typically stunted but remain upright. Cotyledons and first true leaves are yellow or purplish. Root systems will initially be a gray colour and then turn dark brown as the disease develops. Infected plants have an under developed root system with very few lateral roots. (Figure 1

2. Established Stands

Classic symptoms in established stands are patches of stunted, yellow plants. Look for the absence of the fine, fibrous roots and root hairs, with root pruning. Lateral roots are often rotted and even absent. The lack of a healthy root system reduces water and soil nutrient uptake. Roots show reduced rhizobia nodulation resulting in symptoms of nitrogen deficiency.

Established stands that survive the initial infection are typically lacking vigour, thin, yellow and weedy. Affected alfalfa fields appear short, stunted and yellow in characteristic “oval-shaped” patches, often on slopes. There appears to be a definite separation between affected patches and normal plants. Because of the stunted root system, infected alfalfa stands do very poorly during extended dry summer weather, when these patches are particularly noticeable. Plants are slow to break winter dormancy, slower to develop in the spring, lack vigorous growth, and slower to regrow after cutting with poor yields for the life of the stand. (Figures 2 and 3)

No Fungicide Seed Treatment For Aphanomyces In Canada

Fungicides containing metalaxyl (ie Apron ®) are active against phytophthora and pythium, but not aphanomyces. Pyraclostrobin (ie Stamina ®) is a new fungicide seed treatment used on alfalfa in the US to control aphanomyces, but is not registered for this use in Canada. Pyraclostrobin is the active ingredient in the foliar fungicide Headline ®.

Sometimes Confused With Sulphur Deficiency

The yellow stunted growth of ARR is sometimes confused with sulphur deficiency. If in doubt, tissue testing is a good diagnostic tool for sulphur deficiency to rule it out. Alternately, apply some sulphate to a test area of the field. If alfalfa plants green up following rain the stunting and yellowing was due at least in part to sulphur deficiency. http://fieldcropnews.com/2014/07/sulphur-on-alfalfa/

Highly Resistant Variety “Foundation APR”

There is nothing that can be done to remedy a stand infected with aphanomyces. Aphanomyces is best managed by using resistant varieties, similar to what has been done with phytophthora root rot resistant alfalfa varieties. (Figure 4) The first aphanomyces resistant varieties were developed in the 1980’s. However, genetic variations (Race 2) of aphanomyces have challenged this resistance and the original Race 1 resistant varieties have become less effective. More recently, Race 2 resistant alfalfa varieties were developed. Race 2 aphanomyces has been confirmed in Ontario. Race 2 resistant varieties are also resistant to Race 1. Many currently marketed alfalfa varieties are resistant to Race 1, but fewer are resistant to Race 2. If variety resistance to Race 2 is not specified, assume it is only Race 1 resistant. Aphanomyces can be most effectively managed by the use of a highly resistant Race 2 alfalfa variety, such as the “Foundation APR” newly available from Quality Seeds.

Foundation APR is an excellent performer in poorly drained or variably drained soils, with excellent yields, forage nutrient quality, persistence, fine stems, and early to medium maturity that is suitable for both hay and haylage. There will be limited supplies of Foundation APR available in 2016, with a full launch in 2017.


Figure 1 - Aphanomyces root rot of alfalfa seedling. (Photo courtesy of University of Wisconsin).


Figure 2 – Aphanomyces Appearance In An Established Stand

(Photo courtesy Of Dr Dan Undersander, University of Wisconsin)


Figure 3 – Aphanomyces Infected Plants (Bottom) Compared To Normal Plants (Top)

(Photo courtesy Of Dr Dan Undersander, University of Wisconsin)


Figure 4 – Aphanamyces susceptible infected variety (left), Resistant variety (right)

(Photo courtesy Of Dr Dan Undersander, University of Wisconsin)