Corn, Soybean, Wheat, and Alfalfa Field Guide

ALFALFA WEEVIL

Larvae		Damage

Identification & Incidence: Overwintering adult weevils become active during the first warm days of spring and deposit eggs, which hatch into larvae that may defoliate the 1st cutting. Larvae pass through 4 instar stages ranging in size from 1/8 to1/2 inch and then pupate in a fibrous pupal case before transforming to adults. Life cycle includes one generation per year in Ohio.

Sampling: A larval count is made by collecting 10 stems from a location and shaking the stems in a bucket to dislodge the larvae. This is repeated until 30 stems have been sampled.

Economic Threshold: Treatment is based on the stand height, tip feeding and number of larvae per stem.

ALFALFA INSECTICIDES¹

PHYTOPHTHORA ROOT ROT

Description: Plants are stunted with yellow, red, or purple leaves. Dark brown to black lesions occur on tap roots. Frequently the lower portion of the tap root is rotted off immediately below the crown region. Phytophthora also causes damping off, with seedlings failing to emerge or collapsing at the soil surface.

Location: Phytophthora root rot occurs throughout Ohio. It is especially severe on heavy, poorly-drained soils. Symptoms are frequently seen in low areas or between tile lines, while plants in drier areas appear healthy.

Time of Attack: Infection can occur any time during spring and summer the soil is water-saturated.

Management:

• Resistant varieties
• Seed treatment
• Improve soil drainage

SCLEROTINIA CROWN AND STEM ROT

Description: Soft mushy stems with masses of cottony growth are clear signs of Sclerotinia crown and stem rot. Hard, black sclerotia, 1/8 to 1/4 inch in diameter form on diseased stems and crowns.

Location: The disease occurs throughout Ohio, but is most common in late summer (August) seedings, especially when minimum tillage methods are used.

Time of Attack: Infection by Sclerotinia takes place in the fall. Plants die throughout the winter and spring, with symptoms most conspicuous in April and early May. Symptoms are rarely seen after the first harvest.

Management:

• Early August or spring planting
• Till problem field before planting

LEPTO LEAF SPOT

Description: Leaf lesions begin as pinpoint-sized dark spots. Older lesions are oval with tan centers and dark brown borders. Most lesions are surrounded by a yellow “halo.” Heavily infected leaves turn brown and drop.

Location: Lepto leaf spot occurs throughout Ohio.

Time of Attack: Infection can occur throughout the growing season whenever foliage remains wet for an extended period.

Management:

• Resistant varieties
• Cut frequently

VERTICILLIUM WILT

Description: Symptoms usually do not become obvious until the third year after seeding. Affected plants are scattered throughout a field. Upper leaflets turn yellow or pink and often curl or twist. Stems are stunted but remain erect.

Location: Verticillium wilt has been found in 17 counties in central and northeastern Ohio. It has not been reported south of Interstate 70.

Time of Attack: Verticillium wilt usually spreads within a field and to neighboring fields on infested harvesting equipment. Infection can occur throughout the growing season.

Management:

• Resistant varieties

SPRING BLACK STEM

Description: Black lesions occur on lower leaves and stems. The entire lower portion of the stem may be blackened and brittle. Heavily infected leaves turn yellow and drop.

Location: Spring black stem occurs throughout Ohio.

Time of Attack: Infection takes place primarily during cool, wet periods in the spring and fall. Most damage occurs in the spring, prior to the first harvest.

Management:

• Monitor soil pH and fertility
• Frequent cutting

ANTHRACNOSE

Description: Diamond-shaped lesions occur near the base of the stem. Lesions are tan with dark brown borders. Infected stems wilt, producing the characteristic “shepherd’s crook” symptom. When crowns are invaded the inner tissues turn bluish-black and the plant dies.

Location: Anthracnose occurs throughout Ohio on susceptible alfalfa varieties.

Time of Attack: Anthracnose occurs during relatively hot weather, from June through September. The spores are spread from plant to plant by splashing rain or by means of infested harvesting equipment.

Management:

• Resistant varieties

RELATIVE EFFICACY OF SEED TREATMENTS FOR CONTROL OF CERTAIN DISEASES OF ALFALFA IN OHIO

ALFALFA STAND EVALUATION

A healthy and vigorous alfalfa stand minimizes many production problems. Evaluate the stand and its yield potential by counting the number of stems per square foot. Count stems that will be cut by a mower, after plants are 4 to 6 inches tall.

Dig a random sampling of plants and split roots lengthwise to assess their health and vigor. Healthy stands have fewer than 30% of plants with significant injury (discoloration and rot in the crown and taproot). Stands with a high percentage of plants with greater than 50% rot should be considered for replacement.

The following table provides general guidelines for plant density in alfalfa stands.

Generally, stands with less than 40 stems per square foot or 3 to 4 healthy plants per square foot will not be profitable for mechanically harvested forage production.

ALFALFA RESEEDING GUIDELINES

Recommendations for reseeding alfalfa stands vary according to age of the stand. This is because older alfalfa stands have a buildup of pathogens and release compounds that are toxic to new alfalfa seedlings (autotoxicity).

Reseeding in the First Year

• Disk down or kill a seeding failure and reseed either in late summer after a spring seeding attempt or the following spring after a late summer seeding attempt. Autotoxicity is not a problem in the first year.
• Reseed gaps in the stand as soon as possible in the first year.

Reseeding After Older Alfalfa Stands

• The best practice is to rotate out of alfalfa for a full year or more.
• Do not interseed alfalfa to thicken a stand that is 2 years of age or older. Autotoxic compounds and competition from existing plants prevent successful long-term results. Interseed red clover or grass to lengthen life of the forage stand.
• If crop rotation for a year is not a viable solution, kill the alfalfa stand in the fall and seed the next spring. A second alternative is to kill the alfalfa stand in the spring, rotate to a summer annual crop, and reseed to alfalfa in August.
• Research demonstrates that yields will be lower where the interval between alfalfa stands is less than one year.

ALFALFA HARVEST MANAGEMENT

Timely harvest management is critical to obtaining high yields of high quality forage while ensuring stand persistence. The desired forage quality goal should be determined and used as a basis for harvest timing.

A method for estimating fiber content (an excellent measure of alfalfa quality) of the standing crop is described below.

When timothy or smooth bromegrass are included in the stand, do not cut until the grass is in early heading stage to prevent damage to the grass stand. These grasses are best adapted to a 3-cut schedule with alfalfa. Mixed stands with orchardgrass, ryegrass, tall fescue, and reed canarygrass can be harvested on a 4-cut schedule.

Allow alfalfa to reach early to mid-flower stage during one growth interval to improve stand persistence. This should occur in late summer after the second or third harvest), because forage quality does not decline as quickly at this time of the year with advancing maturity as earlier in the year.

ESTIMATING ALFALFA QUALITY IN THE FIELD

1. Choose a representative 2-square-foot area in the field. Determine the maturity of the most mature stem in the sampling area (vegetative, bud, or flower).
2. Measure the length of the longest stem (from soil surface) in the 2-square-foot sampling area. Pull the stem to its full length for an accurate measurement.
3. Use the chart on the next page to determine estimated neutral detergent fiber (NDF) of the standing alfalfa crop. Example: Longest stem is 28 inches, most mature stem has buds; NDF = 38%.
4. Repeat steps 1 to 3 in four or five representative areas across the field and take the average.

NOTE: This procedure does not account for changes in quality due to harvesting and storage. These factors may further raise NDF content by 3 to 6 units or more. Therefore, begin harvesting when the NDF of the standing forage is 4 to 6 units lower than the desired goal. Mixed alfalfa-grass stands should be harvested even earlier.

This procedure is most accurate in healthy stands of pure alfalfa not under stress.

ESTIMATING ALFALFA NDF IN FIELD

FALL HARVEST TIMING

Reduce the risk of winter injury by making the last harvest of alfalfa by September 7 in northern Ohio and by September 15 in southern Ohio. Cutting during late September and October disrupts the alfalfa’s ability to accumulate energy and protein reserves used for winter survival and spring regrowth.

Sometimes cutting in the fall cannot be avoided. Below are strategies to reduce the risk of stand injury and yield reductions the next year.

Reducing Risk of Fall Cutting

• Limit fall cutting to stands where harvest intervals during the growing season were long or allow a growth interval of 45 days before a fall harvest. A 3rd cutting during the fall is less risky than a 4th or 5th cutting.
• Maintain optimal soil fertility levels (pH, P and K) to keep the stand healthy.
• Disease resistant, winter hardy varieties will lessen the risk of injury from fall harvesting.
• Do not cut fields that suffered significant stress (wet soils, potato leafhopper injury, etc.) during the growing season.
• Cutting after killing frosts (25°F for several hours) in late October or early November is an option for well-drained soils not prone to heaving. Leave a 6-inch stubble. Late removal of plant cover increases the risk of frost heaving on soils prone to heaving.
• Do not harvest late summer seedings in the fall.

NUTRIENT DEFICIENCY SYMPTOMS IN ALFALFA

Nitrogen

• deficiency due to poor nodulation
• plant is stunted and has a light green appearance, rounded leaflets with yellowing on tips
• firing on lower leaves first, develops into necrosis (brown tissue), eventually leaves will fall off (abscise)

Phosphorus

• plants are dark green, spindly and stunted
• petioles and leaflets are tilted upward, lower leaves may show firing
• stems may turn red
• reduced nodulation

Potassium

• stunted plants
• yellow to white spots along leaf tips and margins

Calcium

• delayed emergence of primary leaves which when emerged may appear cup shaped, and necrotic
• chlorotic bands appear around the rest of the leaves
• terminal buds deteriorate and petioles break down

Magnesium

• interveinal chlorosis in which the base and lower center of the leaf are not affected.
• downward curling of leaf margins and yellowing from the margin inward

Sulfur

• leaves including veins turn pale green to yellow
• young leaves are affected first
• terminal buds remain alive

Manganese

• interveinal chlorosis (not always distinct), buds remain alive
• spots of dead tissue appear on the leaves

Iron

• interveinal chlorosis, buds remain alive
• sharp color contrast between veins and the leaf
• spots of dead tissue appear on the leaves

Boron

• stunted growth, distortions at the tips or base of young leaves, terminal bud is dead
• leaves near the growing point are yellowed and sometimes reddened, while lower leaves appear healthy (sometimes confused with “hopper burn”)
• internodes are shortened, stems cracked, buds can be absent, distorted, light brown in color or dead
• pink to whitish-yellow spots at leaf base

Copper

• young leaves may wilt, wither and shed without appearing chlorotic

Zinc

• growth is stunted
• interveinal chlorosis, brown spots, and dead tissue

Molybdenum

• see nitrogen deficiency (same deficiency symptoms)

POTASH (K₂O) RECOMMENDATIONS FOR ALFALFA AT VARIOUS YIELD POTENTIALS, CEC'S & SOIL TEST LEVELS.

PHOSPHATE (P0₂O₅) RECOMMENDATIONS FOR ALFALFA

NUTRIENT SUFFICIENCY RANGES FOR ALFALFA

(Top 6 inches sampled prior to initial flowering)