CAUSES OF WET WEATHER ALFALFA STAND LOSSES

Why do we lose alfalfa stands during wet weather? Two entirely different things happen to alfalfa when the soil stays wet for extended periods of time - scald and root rot.  

Scale occurs when alfalfa plants stand in water while the sun shines brightly. This was frequently what happens during May or June after rainy springs. After several weeks of rain, fields are wet. Then the sun comes out brightly.  

Generally what happens is that the standing water acts as a magnifying glass and concentrates the sun's heat. The standing water also stopped the soil from cooling. The temperature shoots up very high and the plants die very quickly. They frequently dies so fast that the leaves do not have time to turn yellow or fall off. During these conditions, we see brown stems with brown leaves still attached for several days. Within a few days the dead alfalfa crown and root rot and can pulled up easily.  

Scale is much less frequently observed if the water is running across the field. Running water does not get so hot, as most of us know who have walked in water barefooted.  

Scale is much less severe if the alfalfa plants have a good canopy of leaves shading the soil and standing water. Again the soil does not get as hot under the leaves and stems.  

Root Rot also causes stand loss during extended periods of wet soils. Root rots are caused by fungi, sometimes called water molds. For root rot to kill alfalfa plants, three things must occur:  

1. One of the fungus organisms must be present in the soil. They are always present in nearly all our soils and there is practically nothing we can do about it.  

2. The soil temperature must be between 60⁰ and 85⁰. That is why we don't observe root rot during the winter when soils are frequently saturated for weeks.  

3. The variety of alfalfa must be susceptible to the root rot. All good varieties have resistance to one of the most important fungus diseases - - Phytophthora Root Rot. Some also have resistance to a related disease - - Aphanomyces Root Rot. It is not in all our alfalfa soils; however, some varieties have a little resistance to this fungus. Another water mold, Pythium, frequently attacks alfalfa seedlings. No variety has good resistance to this disease, but some seed treatments can suppress the disease for a few weeks after planting.  

There are other ways that alfalfa is harmed by excess soil water but they normally decrease yield for a period of time and do not kill the plants.  

The most important way to combat stand losses due to excess water is to choose soils that allow water it infiltrate rapidly and to provide drainage - - either good surface drainage or tile drains. See OSU Extension Facts 2089, Alfalfa Stand Establishment, for additional information about site selection for new alfalfa stands.