Alfalfa fields may periodically exhibit yellowing of foliage. There are several possible causes for such symptoms.
Leaf spot diseases. During springtime, several leaf spotting diseases–including Lepto leaf spot and summer black stem & leaf spot—are common in alfalfa. Very wet weather in spring and early summer favor activity of leaf spotting diseases in first and second cuttings. Wet and humid weather during summer favor other leaf spotting and blighting diseases. In all such cases, leaf spots and blights weaken the plant but alfalfa often outgrows the damage in later cuttings. Maintain a regular cutting schedule, cutting at 30- to 35-day intervals.
Potato leafhopper. Potato leafhoppers are common in Kentucky alfalfa fields. Information on recognition, scouting, and control are available from UK’s Entomology Extension program. See our previous article or your county Extension office.
Soil compaction. Wet soils this spring during preplant operations or hay harvesting operations can result in severe compaction in some fields. Check for soil compaction by digging and examining both root systems and soil structure. If the compaction is so severe that the taproot cannot pass through the compacted zone, yields will be reduced significantly and plowing and replanting might be the only option. Remember, it is much easier to prevent than to alleviate soil compaction.
Potash deficiency. High quality alfalfa removes a large amount of potassium from the soil each year. Soil test K levels should be monitored closely and fertilizer K should be applied whenever it is recommended. It is possible that some plants in your field may exhibit mild potash deficiency symptoms even if potash levels in the soil are adequate, since roots that are limited by compaction and/or root rots will be less effective at taking up potash. Maintaining soil test levels and preventing soil compaction will help to assure maximum productivity and stand longevity.
Root rots. There are a variety of root-rotting diseases of alfalfa that are favored in the saturated soils. The most damaging is Phytophthora root rot; which can attack any part of the root system of plants of any age. Aphanomyces and Pythium organisms are also known to attack the fine feeder roots of mature plants of alfalfa when soils are saturated. Always select varieties with R or HR ratings to Phytophthora and Aphanomyces root rots when seeding alfalfa in Kentucky. Unfortunately, there is no known resistance in commercial cultivars to Pythium infection, but improving soil drainage and minimizing soil compaction will help with all three diseases.
Poor nodulation. Check nodulation of new seedings by carefully digging and washing root systems and examining for nodules. Poor nodulation of roots may be the result of root-rot infections or of poor viability of the Rhizobium bacterium on the seed. If poor viability on the seed is the cause, an inexpensive practice to improve the chances for nodulation can be found at: http://plantpathology.ca.uky.edu/files/ppfs-ag-f-04.pdf.
Crown rot diseases. Dig plants and cut into the crowns. Those that are showing brown discoloration are exhibiting crown rot. A variety of soil-boirne fungi can cause crown rot. Adapted varieties of alfalfa can sometimes recover from crown rots. However, if severe, crown rots can be a significant problem for long-term health of the stand. Thus, if you see a high frequency of crown rot in a particular field, that is usually a flag to rotate.
Probably the best indication of when to rotate is stand density. Approximate guidelines as to economically acceptable stands from Dr. Garry Lacefield, UK Forage Agronomist, are:
3 crowns per square foot for hay
1 crowns per square foot for grazing
Dr. Lacefield points out that these are approximate guidelines. For example, a beef cattle producer often will meet his/her production goals well with a much lower density of alfalfa crowns than a hay producer. He also indicates that, for the Upper Midwest, for high-quality dairy hay productions, the standards are based on stem density, since this more closely correlates to forage production than crown density.:
55+ stems per square foot: no yield reduction
40-55 per square foot: some yield reduction
Below 40 per square foot: give consideration to replanting
If stands are less than needed for your yield goals, plan a rotation away from alfalfa followed by re-seeding.
~ Drs. Paul Vincelli, Chris Teutsch and Kiersten Wise, revision from an early extension article on this topic by Paul Vincelli and Greg Schwab.