If you notice the leaves of your alfalfa plants turning yellow, it can be cause for concern. Yellowing leaves on alfalfa indicate the plant is stressed and not receiving the proper nutrients and growing conditions Identifying and correcting the cause is key to restoring the health and productivity of your alfalfa crop
There are several potential reasons alfalfa leaves may turn yellow:
Nutrient Deficiencies
Lack of key nutrients often results in yellowing alfalfa leaves. The most common nutrient deficiencies that cause yellowing are
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Nitrogen – Nitrogen is essential for overall growth and green color. Uniform yellowing of older leaves first indicates nitrogen deficiency. Apply a nitrogen fertilizer to remedy.
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Sulfur – Plants low in sulfur resemble nitrogen deficiency. Yellowing starts on younger leaves. Use a sulfur-containing fertilizer.
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Phosphorus – Phosphorus aids root growth and energy transfer. Yellowing leaves with purplish tint signals deficiency. Apply phosphorus fertilizer.
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Potassium – Needed for photosynthesis and disease resistance. Yellowing leaves with scorched margins points to potassium lack. Use potash supplements.
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Iron – Iron aids chlorophyll synthesis for green color. Interveinal yellowing of newest leaves is a symptom. Foliar iron sprays can correct this.
Water Stress
Too little or too much water can stress alfalfa and cause yellowing. Ensure consistent soil moisture by:
- Watering when top few inches become dry
- Providing supplemental irrigation in dry periods
- Avoiding waterlogged soil
Pest Issues
Pests that feed on alfalfa can lead to yellowing when populations are high:
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Aphids – Cause yellow stippling and leaf curling. Use insecticidal soap sprays.
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Leafhoppers – Create yellow v-shaped lesions. Apply registered insecticides.
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Spider Mites – Result in stippling and bronzing. Release predatory mites.
Diseases
Fungal diseases thrive in wet conditions, damaging roots and foliage:
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Rust – Causes yellow spots on upper leaves. Apply fungicides preventatively.
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Leaf Spot – Creates round yellow lesions. Cut early to remove inoculum.
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Root Rots – Lead to yellowing and stunting. Improve drainage.
Soil pH Imbalance
Soil pH outside the ideal range of 6.5-7.5 locks up nutrients, causing deficiency. Test and amend soil pH if needed.
Compaction
Compacted soil restricts root growth, reducing nutrient uptake and causing yellowing. Avoid traffic on wet soils. Subsoil if necessary.
Cutting Too Early
Cutting before sufficient regrowth depletes root reserves, stressing plants. Stagger cuttings and don’t harvest too low.
Freeze Damage
Late spring freezes can damage young alfalfa growth, causing leaf yellowing. Provide insulation like hay mulch if cold is severe.
Testing is Key
If the reason for yellowing alfalfa is not obvious, laboratory testing can help pinpoint the cause:
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Soil testing detects pH and nutrient issues.
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Plant tissue analysis identifies specific deficiencies.
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Pathogen testing diagnoses disease problems.
Preventing Yellowing
Proper care and maintenance helps prevent leaf yellowing:
- Test soil and follow fertilization guidelines
- Control pests promptly
- Irrigate adequately during droughts
- Improve drainage in wet areas
- Avoid cutting or traffic on wet fields
- Monitor soil pH and temperature
While yellowing leaves on alfalfa can be alarming, identifying and addressing the cause through improved practices can restore plant health. Consult local Extension resources for help diagnosing and correcting yellowing issues in your alfalfa field. Healthy, green alfalfa means higher yields and quality.
Yellow, stunted alfalfa may not be entirely due to drought
Fred Springborn, and Christina DiFonzo, , Department of Entomology – July 25, 2012
Potato leafhopper numbers are above threshold levels in some alfalfa fields around Michigan.
Livestock producers concerned about the availability of hay and haylage this year need to scout and potentially treat alfalfa fields for potato leafhoppers. Potato leafhoppers damage alfalfa plants by piercing the leaf surface with its stylet and sucking out plant juices. As it probes, potato leafhoppers injects a salivary toxin that affects plant cells and disrupts vascular tissue. The disruption and blockage of the vascular system is what leads to the first visible sign of potato leafhopper damage, the yellowing of leaf tips, often referred to as hopper burn (Photo 1). The end result is a reduction in tonnage and quality. In severe cases of potato leafhopper feeding, whole plants and whole fields will turn yellow and be severely stunted (Photo 2).
Each year, potato leafhoppers blow in from southern states on storm fronts and are deposited in Michigan. Populations are variable from year-to-year, depending on timing of arrival and number, and how rapidly they reproduce once they arrive. In typical moist, humid Michigan summers, potato leafhoppers are controlled naturally by a fungal pathogen that infects and kills the insect. The potato leafhopper population often crashes by the middle of August due in large part to the fungal pathogen.
Unfortunately, the hot, dry weather we have experienced this season favors potato leafhoppers. The high temperatures allow leafhoppers to multiply quickly. The plant sap of drought-stressed plants is often more concentrated and nutritious for the insect. Meanwhile, biocontrol is less, as the fungal pathogen is not active under low humidity. All of this results in potentially large potato leafhopper populations in fields. At the same time, stressed plants are less able to compensate for sap removal and the effect of the toxin, so the impact of potato leafhopper feeding is magnified.
Potato leafhoppers are small and highly mobile and they are sometimes difficult to see and count (Photo 3). Threshold levels are not detectable by driving by the field or from the seat of a tractor or ATV. Alfalfa fields that are at threshold may show few signs of damage (Photos 4 and 5). In alfalfa, a sweep net is recommended to sample potato leafhoppers (Photo 6).
Alfalfa thresholds are based on a combination of the number of potato leafhoppers per 100 sweeps and the height of the alfalfa.
- Below 3 inches = 20 adult potato leafhoppers per 100 sweeps
- 3 to 8 inches = 50 adult potato leafhoppers per 100 sweeps
- 8 to 12 inches = 100 potato leafhopper adults or nymphs per 100 sweeps or one per sweep
- Over 12 inches = 200 potato leafhopper adults or nymphs per 100 sweeps or two per sweep
Tall alfalfa that is over threshold can also be managed by timely cutting. Cutting is preferred because it may eliminate the need for insecticide use and avoids issues with preharvest intervals. Be aware that neighboring fields of alfalfa and other crops will likely increase in potato leafhopper numbers as adults leave the cut field. Regrowth should be scouted and closely monitored after cutting, as the threshold is lower for shorter alfalfa. If treatment of regrowth is necessary, insecticides applied to the shorter stand will have better coverage.
Potato leafhoppers are relatively easy to control with insecticides and there are many registered products, including inexpensive choices. Note labeled preharvest intervals and warnings for applications if honeybees are nearby.
While drought is the dominate factor limiting alfalfa regrowth in several parts of the state this summer – southwest Michigan in particular – there is still moisture available for alfalfa regrowth in many areas. Controlling potato leafhoppers will help alfalfa make the most of the limited moisture that is available and maximize alfalfa yield and quality in the face of a short hay supply.
Additional information:
- MSU Extension’s Drought Resources
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