Alfalfa disease update – April 15, 2024
The wet and cooler weather conditions experienced in Oklahoma during the last months of 2023 and the first months of 2024 favor the development of plant diseases that many growers are unfamiliar with. One example is the diseases caused by plant pathogens affecting alfalfa, which can easily go unnoticed during the season if the growers do not frequently scout the fields.
One of the most destructive diseases of alfalfa is Sclerotinia crown and stem rot, aka “white mold”. The pathogen that causes this disease (Sclerotinia trifoliorum) infects the alfalfa seedlings in the fall right after planting. However, symptoms may not become visible in the field until the following spring. Alfalfa fields seeded during spring are less susceptible to infection by the pathogen since stands can develop stronger and larger crowns before the infectious period by the pathogen. In contrast, alfalfa fields seeded in late summer or early fall are most susceptible to the disease. Symptoms of the Sclerotinia crown and stem rot have already been reported in alfalfa fields located in the Central part of Oklahoma in April 2024. In dry winters, this disease is not a problem.
Symptoms: Symptoms of the disease include the development of profuse, fluffy, white mycelium on the base of the alfalfa stem, which is visible under cool and moist conditions (Figure 01). Diseased alfalfa stems have a bleached, dry appearance. Infected plants become wilted and eventually collapse in the field due to the stem and crown rot, which softens and weakens the stem (Figure 02). The fungus also produces black, hard structures, known as sclerotia, formed when no food is left (e.g., the plant is already dead) or the environmental conditions become unfavorable (e.g., too hot or too dry) (Figure 01). The fungus uses these structures to survive in the soil for many years in the absence of the plant host. Symptoms of this disease can also be mistaken for winterkills.
Disease cycle: The sclerotia of the pathogen oversummer in the soil. In the fall, with the soil moisture and decreased soil temperatures (50 – 59 °F), the sclerotia will germinate and produce structures known as apothecia and spores. The apothecia are a small mushroom-like structure, while the spores are considered the “seeds” of the fungus. The spores will be blown to nearby plants, and under wet and cool conditions, the infection process in the alfalfa plants will occur.
Disease Management
Planting dates: A primary cropping practice for preventing alfalfa crown and stem rot is planting the crop in the spring or late summer rather than the fall. Warmer temperatures at these times inhibit germination of the sclerotia in the soil, allowing the plant to grow bigger and stronger when the pathogen's spores are released in the fall.
Crop rotation: The pathogen that causes Sclerotinia crown and stem rot only infects alfalfa, clover, and related forage legumes. The pathogen that causes disease in alfalfa (S. trifoliorum) is not the species that attacks soybean, peanuts, tobacco, canola, or vegetable crops. Field crops (e.g., corn, cotton, wheat, sorghum) and annual forage grasses (e.g., Sudangrass) may be used in crop rotation. However, the growers need to keep in mind that the sclerotia of the fungus can survive in the soil for up to 5 to 6 years without any host plant. Once the pathogen is well established in a field and the soil is highly infested with sclerotia, crop rotation may be of less value because of the long survival time of these fungi structures.
Weed control: The pathogen can remain on volunteer clovers in a pasture indefinitely. Weeds, such as chickweed, further encourage disease by prolonging moist conditions in the canopy. Other weed hosts (e.g., pineappleweed, sowthistle, groundsel, mayweed, mustards, radish, and legumes) can also be the source of new infections. Good weed control reduces the potential survival of the fungi and helps to reduce humidity in the canopy, which favors disease development.
Canopy management: In fields where the disease had already been reported, strategies to manage the disease included reducing the number of seeds planted per acre to reduce the humidity in the crop canopy. Because infection by spores of the fungus requires an extended period of free moisture, orienting rows parallel to the direction of the prevailing winds may be of some value in drying the canopy quicker after rain or irrigation events.
Host resistance: Until now, no effective genetic-based resistance has been incorporated into commercial varieties.
Fungicide applications: Currently, one fungicide premix, pyraclostrobin/boscalid (Pristine), is known to reduce disease severity and increase yield in the first cutting. However, application timing is challenging. Mixing two active ingredients with different modes of action (pyraclostrobin and boscalid) is more expensive but provides better disease control and should slow the development of resistant Sclerotinia isolates than using a strobilurin (pyraclostrobin or azoxystrobin) by itself. Please contact your county Extension agent for more information on fungicide applications.
Figure 1: Characteristic symptoms of Sclerotinia crown and stem rot, with the development of profuse, fluffy, white mycelium on the base of the alfalfa stem. The yellow arrows indicate the presence of sclerotia, hard black structures that allow the pathogen to survive in the soil for many years in the absence of the plant host.
Figure 2: Symptoms of alfalfa plants affected by Sclerotinia crown and stem rot. Diseased alfalfa stems have a bleached, dry appearance. Infected plants become wilted and eventually collapse in the field due to stem and crown rot, which softens and weakens the stem.