Sunflower Pest Management (cont.)

[Insects] [Diseases] [Weeds] [Birds] [Other]

Other Pests and Damage

Several sources of sunflower injury exist. Some of them are confused with damage from insects or diseases.

Rabbits

Rabbits will start foraging soon after seedling emergence, especially near the edges of fields. They will tend to concentrate on one row and apparently eat their fill, then leave until the next feeding period. Continued feeding by rabbits has been observed until the plants are 8 to 10 inches tall. Rabbit feeding on such large plants may be confused with deer. However, deer can be detected by their tracks.

Deer

Deer begin foraging on sunflower plants when the plants reach 8 to 10 inches and continue through harvest. They feed in areas near cover, such as wooded areas. All leaves of young plants will be consumed below the growing point. Heads will be foraged until near maturity and seeds until harvest. Often deer will knock down the stalk to facilitate foraging.

Gophers and Mice

Gopher and mouse damage is usually seen just after planting. It generally occurs next to overgrazed pastures, grassland recently converted to cropland, and fields next to abandoned areas. The seed will be dug up, split open with the kernel consumed, and the hull left on the soil. Several seeds in a row will be eaten. Seedlings are eaten occasionally when they are 2 to 3 inches tall. If the growing point is consumed the seedling gradually dies. Shooting or rodenticide treated oats will control gophers and mice.

Lightning

Lightning damage is sometimes mistaken for a disease. It is distinguished from disease damage by the sudden death of the plants in the affected area and the fact that both sunflower and weeds (not grass, however) are killed (Figure 87). Near the edge of the area plants are wilted but not dead, and the stalks may have a brown to blackened pith. The area may be as large as 50 to 100 feet in diameter. The affected area usually is circular-shaped and does not increase in size after the first two weeks. Flags may be placed at the edges of the affected area to observe if the damage gradually progresses beyond the flags. If damage does gradually extend beyond the flags, this could indicate damage from a source other than lightning.

Flooding

Soils should have good drainage for sunflower production, but the crop doesn't differ greatly from most other crops. In flooded sunflower, it was found that ethylene increased in the stems and roots below the water. Later chlorophyll breakdown and leaf epinasty resulted. Sunflower plants flooded longer than three days may not recover. Cool cloudy days during the flooding period reduces the damage, whereas hot and sunny days may hasten the death of plants.

Heat Canker

Warm temperatures and sunny days can result in heat canker injury to young sunflower seedlings growing in black or dark moist soils. Hot temperatures at the soil line cause cell death in the young stem and the plants will show bands of yellowing and constricting. In severe cases, the constricted area completely girdles the stem at the soil line and the plant topples over. The sunflower seedling will not recover since the growing point is above this site. Plant populations can be reduced significantly in some cases.

Frost Damage

Sunflower seedlings in the cotyledonary stage (VE) can withstand temperatures down to 25 degrees Fahrenheit when just emerging from the soil. Sunflower in the V, V-2 and V-3 stages become less tolerant to frost as they grow and develop. The terminal bud can be frost damaged to seedlings with two, four, and six true leaves.

Sunflower is most susceptible at the bud (R-4) and pollination stages (R-5) of development. Temperatures of 30 F or less can cause damage to the anthers and stigma's of the pollinating disk flowers.

Sunflower has a composite type flower. Several rows of showy yellow ray flowers encircle the head and are commonly called the "petals," though each is an individual flower. The center portion of the head, and by far the greater part, is composed of inconspicuous individual flowers, one for each seed that may develop. These disk flowers mature in circles from the outside of the flower head to the center, so that at various stages the disk flowers ready for pollination appear as a yellow circular band in the brownish or dark center of the head. It is these disk flowers that are sensitive to frost.

The result of the frost damage in the flowering period is circular bands of undeveloped seed that would vary with individual flower heads from a band around the outside edge to an area in the center. Unopened buds are less susceptible to frost than the opened flower heads. Growers can determine the extent of injury by cutting the surface of the flower head.

Once pollination is completed and 10-14 days after petal drying occurs, the sunflower plants can withstand frost temperatures as low as 25 F and have only minor damage. Twenty-five degrees Fahrenheit at the bud stage will often damage the stalk below the bud and seeds will not develop. If hard frosts do occur, many times only the seed in the center of the head (the last to pollinate) will be affected.

When sunflower heads start to turn yellow on the back side and the bracts are drying and turning brown, most risk of frost damage is very minimal.

In nonoilseed sunflower, frost damage can cause quality problems by causing a dark brown to blackened nutmeat to result during the roasting process. For the birdseed market, light-weight sunflower seed and brown colored seeds are the result of frost damage and will be discounted.

For oilseed sunflower, reduced test weight per bushel and lower oil percent may result from a frosted immature sunflower crop.