Few California soils in sunflower growing areas have shown yield limiting K deficiencies; sandy soils are the most likely to show K fertilizer responses.Sulfur deficiency, though rarely observed in sunflowers, is characterized by slow growth and overall uniform light green color of the plant. It is generally only observed once every 5 to 7 years, and may occur only after high rainfall, prolonged wet soil conditions, and cooler soil temperatures from January to March, especially in wheat. In the Sacramento Valley, some soils may show temporary deficiency of S, and most irrigation water contains little or no sulfur. Fertilizers such as ammonium sulfate, a common source of S in the past, are no longer being used as commonly as non-S-bearing aqua ammonia and other high-analysis fertilizers and therefore S deficiencies might occasionally occur. Broadcast and incorporate elemental S into S-deficient soils at a rate of 50 to 100 pounds of S per acre to provide a correction that will last several years. Elemental S is best applied in the fall when the fields are bedded up to allow time for oxidation to sulfate-S, the form used by plants. The time necessary to oxidize elemental S depends on soil temperature and moisture, as well as on the size of the particles applied. Particle size may cause the timeline to change from a few weeks to several months before the applied elemental S becomes effective. Other materials such as gypsum , which is about 17% S, provide the readily available sulfate form of S and can also be used. These materials should be applied at a rate that supplies about 25 to 50 pounds of S per acre and should be incorporated into the top 2 to 3 inches of soil to be most effective.Although soil moisture can be assessed by using the “look and feel” method, rolling benches for growing soil moisture sensors can provide more accurate information. There are many sensor options available to growers, including tensiometers and gypsum blocks, that are accurate and relatively easy to use and inexpensive.
Tensiometers indicate soil moisture levels by measuring the soil moisture tension, or how strongly water is held onto soil particles. Therefore, low soil moisture tension indicates moist soil, and high soil moisture tension indicates dry soil. Gypsum blocks buried in the soil measure the electrical resistance of water, which can be converted into a soil moisture tension value. Soil moisture tension is usually expressed in centibars . Soil moisture sensors must be installed in areas that are representative of the field; that is, the site must have a soil type that is typical of the field and needs to receive full irrigation coverage. Sensors should be installed to the depth of the active root water uptake zone, which for sunflowers would be 4 feet. Monitor soil moisture at 1 foot, 2 feet, 3 feet, and 4-feet deep to determine when to irrigate and to ascertain the depth or adequacy of an irrigation. It may also be useful to monitor at 5 feet to determine whether excess irrigation water is being applied, causing water to percolate past the sunflowers’ 4- to 5-foot rooting depth. The threshold level for soil moisture in the root zone for sunflowers and irrigation needs depend on soil type, irrigation system , growth stage, and amount of deep moisture in the soil profile. General guidelines for soil moisture monitoring can be given based on limited experience with sunflower production in clay loam soils in the low desert of California. For heavy soils, the upper limit for soil moisture depletion in the top 2 feet of the root zone is 90 to 120 cb. Irrigation should be applied when soil moisture content reaches this threshold level. If soil moisture is depleted beyond this level, the crop will be under stress.
However, roots can extend beyond 4 feet and this threshold could be reached and the crop may not show signs of stress if there is moisture in the deeper soil profile or a shallow water table available up to 6 to 8 feet below the soil surface. In experiments conducted at the UC Desert Research and Extension Center from 2016 to 2018, sunflower plots that were subjected to 60 percent deficit irrigation practices showed no stress and yields were not affected by the reduced water applications due to the presence of a shallow water table 6 to 8 feet below the soil surface.In some years, soil pests that live at or below the soil line, such as variegated cutworm , wire worms , and seed corn maggot , can seriously damage seedlings and cause significant stand losses. These pests tend to be sporadic in time and space; they tend to be more troublesome in wet years when weed vegetation is heavy, and they have a patchy distribution in fields. They are often problematic in the same field year after year, so monitoring and being familiar with the history of the field is important for managing these pests in crop rotations. Use of insecticide seed treatments, such as Cruiser 5FS will help control early-season soil-dwelling pests. Sunflower Moth Sunflower moth, also known as sunflower head moth , is the most serious pest of sunflower in California and other U.S. sunflower growing states. The adult sunflower moth is grayish, ⅜ inches long, and rests with wings clasped tightly to the body, giving it a slender cigar shape . Eggs are difficult to find because they are usually laid at the base of florets in the flower head. The newly hatched larvae are pale yellow but darken to shades of brown with longitudinal white stripes and a light-brown head capsule. Insect excrement and tangled mats of webbing on the flower heads indicate larval activity . Mature larvae drop to the ground on a strand of silk, crawl into cracks in the soil, spin cocoons, pupate, and later emerge as adults.
There can be 3 generations of head moth per year. In California, the sunflower moth likely overwinters as a larva in the cocoon stage in the soil. In colder climates, such as Midwestern states, the moth is migratory. In the Sacramento Valley, the moths begin to emerge in June and are generally most troublesome in July and August. Early-planted fields sometimes escape moth damage, as moths seem to build up on early planted fields and disperse into later planted fields when they reach greater numbers. In the Imperial Valley, plantings are generally early enough that they escape head moth flights, but if one occurs, it would be in May. Significant outbreaks of sunflower moth can occur, with yield losses of 30 to 60 percent and occasionally 100 percent in fields where the moths are not controlled. The only way to effectively and economically manage this pest is through insecticide treatments. There are no effective cultural practices, and bio-control cannot be relied on because the primary brachonid parasitoid wasp in sunflowers cannot readily reach the head moth larvae deep in the florets with its ovipositor or break through the seed shell to reach the larvae and sting them. Sunflower moths can be monitored using Pherocon IIB pheromone traps baited with sunflower moth pheromone lures. Two traps are generally placed along the north and south side edges of fields, taking advantage of the prevailing winds to maximize trap catches. Traps should be monitored weekly, and more often during bloom when sunflowers are most sensitive to damage by the moths. When trap thresholds reach 4 or more moths per night, especially with July and later-blooming fields, the field should be treated with an insecticide to prevent damage and crop losses, cannabis dry racks especially from secondary pathogens. When using insecticides during bloom, it is critically important to protect honey bees to promote bee activity and crop pollination. If an insecticide treatment is needed, spray before hives are brought into fields prior to bloom or early in the morning before bees are visiting flowers. Insecticides that control head moth include Coragen , Warrior or Asana , and XenTari . Coragen does not control adult moths but gives good caterpillar control with recommendations to apply twice, once at the late flower bud stage and again at the very beginning of bloom , to ensure good coverage and protection. Coragen is relatively safe for bees, but applications should be made early in the morning when bees are less active to protect them from harmful effects of sprays.Lygus bugs are small plant bugs with a very distinctive yellow V on their back . They are serious pests of numerous crops, including strawberries, beans, and cotton. In confectionary type sunflowers, feeding damage causes brown spots known as kernel brown spot , reducing the quality of the seed for the snack food industry. However, the impact of Lygus on hybrid seed production is unclear. Lygus feeding damage may reduce sunflower seed germination, a focus of current UC ANR research. Western Flower Thrips Western flower thrips are tiny, slender, light-yellowish insects.
The nymphs are wingless, while adults have clear, slender wings . When they feed, they generally cause leaves to be deformed, but they also leave behind silver patches on the lower side of leaves with tiny black fecal pellets. Plants usually outgrow the problem, just as they outgrow severe leaf tattering from wind damage. However, high numbers of thrips and heavy feeding damage can injure seedling stands if temperatures are high and plants are water stressed, and they may require a pesticide treatment for control to prevent stand loss.Two-spotted Spider Mite Two-spotted spider mites are found in sunflowers, particularly later in the season as the plants senesce. Mites are small, pinhead-sized, oblong, and yellowish with two dark pigmented spots, and they have eight legs . The eggs of spider mites are whitish and spherical and can be seen with a hand lens. Spider mites are usually found on the underside of leaves, with colonies beginning on the lower leaves and moving upward on the plant. Spider mite feeding damage first appears as stippling on leaves. As numbers increase, spider mites spin fine webbing and move rapidly around the plant on the webbed area. Damage in heavily infested plants includes leaf desiccation with a whitish-gray cast and stunted plants. Infestations are usually associated with mature plants, dust along field edges from dirt roads, water stress, and natural senescence and control is generally not needed. Postharvest Storage Pests Sunflower seeds destined for export must be examined to ensure that they are free of stored product insect pests. These include dermestid beetles ; weevils ; lesser grain borer ; sawtoothed grain beetle ; red and confused flour beetles ; and slender and broad horned flour beetles . More information on stored product pests and their control can be found in the pantry pests notes and in Residential, Industrial, and Institutional Pest Control by O’Connor-Marer .California’s dry climate, and rich, irrigated soils provide excellent conditions for growing strong, healthy sunflowers that experience fewer diseases with less severity than sunflowers grown in the main oil-production areas of the Midwest under summer rainfall. Very few of these diseases lead to yield losses in California growing conditions, but many are of quarantine significance and thus would preclude seed from being exported to foreign countries. This could cause serious economic impacts, so it is important to monitor for diseases. The fungal diseases in California sunflower production classified as quarantine status by many foreign countries include downy mildew , rust , and Sclerotinia head and stalk rot . Fortunately, these diseases are very rare in California due to our hot, dry summers. In a 15-year study by Gulya et al. , only three quarantine-type diseases were found in sunflower in California, including rust in 4% of fields, Sclerotinia rot in 2.6% of fields, and downy mildew in 0.4% of fields. Each importing country has different pathogens that are excluded. Since U. S. companies may not know in advance where the seed will be exported, it is imperative to ensure fields are free from as many diseases as possible. If the pathogen involved is soilborne, rotate to a nonsusceptible crop to reduce the disease inoculum in the field. Downy mildew and rust are sunflower-specific diseases, so any crop is suitable to rotate for controlling them. However, for pathogens with a broad host range like Sclerotinia spp., nonhost monocots must be rotated to help reduce the sclerotia soil inoculum. For diseases such as Rhizopus head rot, rotation offers nohelp since the fungus is ubiquitous and persists as a saprophyte on any organic matter.