In addition to diseases, the cause of some disorders of sunflower appears to be varietal and physiological, with no causal diseases yet identified. In one malady observed, the female lines do not drop their heads after pollination. No management recommendations exist for this disorder given that it appears to be varietal and physiological. Foamy head rot of sunflower produces copious white frothy material that exudes from the flower heads, leaf petioles, and stalk . No wounds are obvious, though insects such as stink bugs and ants are often attracted to the sweet, alcohol-smelling frothy sap. Yields and seed quality are reduced, depending on the severity of the problem. This disorder has been observed only in female lines of select varieties, with certain genetics that predispose them to this problem. This condition usually occurs during hot, dry spells with ample soil moisture. Despite extensive testing, no pathogens have been identified as causal agents . Excessive heat may cause splitting or microwounds that allow the ingress of organisms that otherwise would not be harmful, much like Rhizopus head rot following insect or bird damage to flower heads. The foamy disorder was observed in the same female parent lines in 1993 and 2014, where it was fairly uniformly distributed throughout fields. Similar symptoms were found in some female plants in 2017, after a heat wave, harvest drying rack likely in a different parent line compared with earlier years, as damage was not as widespread as occurred in previous years.
Excessive heat waves can also injure sunflowers, especially during the early reproductive stages through bloom . For example, in 2017 in the Sacramento Valley, the high temperatures in mid June were 100o to 106o F for 8 consecutive days during the R1-R4 stage for some fields. Heat damage that followed included flower buds that did not develop or they partially opened , with some seed set, but yields were significantly reduced. Again, the heat damage was observed only in female lines and not in the males. Some parental lines showed more tolerance to the heat wave than others, with less flower damage, indicating differences in parental line susceptibility. There was some indication that heat injury to sunflowers was worse where fields had gravel streaks . This suggests that it is critical to be prepared with good irrigation management practices before heatwaves occur to ensure that sunflowers are not moisture stressed during extreme heat events. Excessively high temperatures at bloom after several heat waves in 2017 also resulted in sunburn on the fully opened female flower heads , followed by a breakdown of tissue that became gelatinous from secondary pathogens.Symptoms were uniform on plants throughout fields where heat related damage occurred, which is a clue that the cause was abiotic or environmental and not due to a disease. In other fields there were empty or undeveloped hulls due to poor pollination from a lack of honey bee activity from the heat wave . Nematodes Nematodes, microscopic, wormlike organisms that feed on plant roots, are seldom a problem in sunflower production .
However, most nematodes found in California have a wide host range, so it is possible that nematode damage could occur. Nematodes that exist in California that might parasitize sunflower include the root knot nematode , which produces galls on the roots , and the lesion nematode , which causes brown, discolored scarring on the roots . If nematodes are suspected, soil samples can be taken and sent to a lab for pest identification. The only economical management tool is rotation away from any susceptible host crop and using resistant plant varieties in crop rotations, such as certain varieties of tomatoes. Weeds Weed control is essential for maximizing sunflower seed yields and quality and for meeting seed certification standards in hybrid sunflower seed production , especially for prohibited and restricted weeds. The most common prohibited weed in California is perennial pepper weed . Canada thistle is another prohibited weed and a problem for sunflowers grown in the Intermountain area . An isolated population of branched broomrape , also a prohibited rated species, was observed in the Sacramento Valley in 2017 and eradication efforts are in progress . Restricted noxious weeds are not allowed in processed seed lots. Noxious weeds with a restricted rating include Italian thistle , slender flowered thistle , field bindweed , and Johnson grass as well as other perennial sorghum species. While the restricted noxious weed species observed in sunflower seed production are smaller seeded and can be effectively removed during seed conditioning, they should be controlled in the field to prevent risk to seed lot contamination. CCIA also requires rouging of corn and castor bean because it can be hard to remove these seeds during conditioning and there is zero tolerance for them in the seed analyses. Weed control in sunflowers begins with knowing the field history and types of weeds in the field. Post emergence herbicide options are limited for broad leaf weed control in sunflowers once the crop has emerged, so avoid fields with weed pressure, especially for the prohibited and restricted weeds listed above.
In addition, knowing the weed spectrum in the field is needed to help determine which herbicides to use at planting. Prepare a good seed bed to ensure a uniform, good stand that will help shade out and compete with weeds. Planting into moisture on subsurface drip systems, where the soil surface remains dry, helps keep weeds under control. Cultivate sunflowers at lay by to help control weeds. Seed companies may also send in roguing crews to manage weeds. Glyphosate is typically used to burn down winter weeds prior to planting, then preplant herbicides are applied for spring and summer weed control. Preplant herbicides are mechanically mixed into the top 2 to 3 inches of the soil with a cultivator. The choice of herbicide depends on the weed complex present in the field. Mixtures of herbicides are often used, with rates highly dependent on the soil texture, as defined in the pesticide label. Sonalan HFP is often used in sunflower production because it provides good control of nightshade . A tank mix of Sonalan and Medal or Treflan provides excellent control of most annual grasses, fair to good control of small-seeded broad leaves including pigweeds and lambsquarters , and fair control of wild oat , but does not control wild sunflower, cocklebur , or wild mustard . Treflan has some soil residual activity and may damage oats or sorghum the following year. Prowl H2O is sometimes used as preplant incorporated herbicide for weed control in sunflower production, depending on the weeds present in the field. For post emergence weed control, Poast and Select Max can be used on grass weeds including cereal grains, corn, and sorghum. Surfactants and AMS adjuvants are recommended to enhance the efficacy of these herbicides. For broad leaf weed control, Shark can be used only as a hooded sprayer application . Clearfield sunflower with imazamox tolerance and ExpressSun with tribenuron tolerance allow for the control of difficult broad leaf weeds, including cocklebur, puncturevine , and wild mustard. When growing varieties with these traits, be aware of plant back restrictions for subsequent crops, such as tomatoes. Volunteer sunflowers can be a problem in subsequent crop rotations, such as wheat. A frost often kills sunflower plants out of season, vertical growing racks but late-germinating seedlings that get buried deep during tillage and grow slowly can escape these events. Destroying the male rows soon after pollination is complete results in less viable seed remaining in the field. However, destroying them too early after pollination will result in any late-blooming females in the field having the potential to receive sunflower pollen from foraging bees exclusively from outside sources, increasing the risk of varietal contamination. In addition, avoid deep tillage if possible after harvest to reduce the frequency of late emerging sunflower volunteers. In small grains, 2,4-D or MCPA can be used for sunflower control. Watch for Clearfield and ExpressSun technology in crop rotations, as these sunflowers will not controlled by imazamox or tribenuron herbicides.Sunflowers are vulnerable to damage by birds because the seeds are exposed and the large head serves as a perch during feeding, and birds favor them.
Although many species of birds feed in maturing sunflower fields, the greatest losses are caused by migrating flocks of blackbirds . Although blackbirds are protected under the Migratory Bird Treaty Act, cultural practices can be used to manage blackbirds, including bird-scaring cannons that produce an extremely loud explosion . Most growers elect to avoid planting in areas where blackbirds frequent, including the Sacramento bypass, which serves as a wildlife area. Some insectivorous birds are beneficial in sunflower fields when they prey on pests such as sunflower head moth. Also watch for bird damage on newly emerged seedlings.Only the female plant rows are harvested; males are chopped and disked after pollination and seed set. At maturity , females are often treated with defoliants or desiccants to dry them down more quickly in preparation for harvest. This reduces exposure to vertebrate pests, such as birds, weathering , variable maturity due to multiple soil types, and harvest issues due to live, green weeds.Harvest aid products used include the herbicide paraquat, a desiccant, as well as sodium chlorate–based defoliant materials . Sodium chlorate materials are contact only and are very effective as defoliants during hot, dry conditions. Paraquat is most effective during cool or cloudy weather when it is better absorbed into the stem and leaf material, allowing it to diffuse more readily into the plant before being activated by sunlight. Never use glyphosate , as this herbicide will affect seed germination when applied to seed production fields. Sodium chlorate is best applied by ground for maximum coverage. Harvesting is subject to the supervision of the county agricultural commissioner, who must be contacted prior to harvest. After the CCIA completes the three required field inspections, they issue a field inspection report. Once the seed company has the report and is ready to harvest, they notify the grower to contact the agricultural commissioner for the harvester inspection. The agricultural commissioner’s office will not inspect a harvester or authorize harvest until they confirm that CCIA has issued the field inspection report. A county agricultural commissioner biologist must inspect the harvester to verify that the harvester has been properly cleaned of any contaminating seed sources . When approved, harvesting is usually done by growers using their own combine and header, when the seed moisture is less than 12 percent. Proper combine setup is essential to prevent seed loss and injury, excess foreign material, and weed seeds in the harvested product.Sunflower seeds are hauled to the warehouse for a precleaning, also known as scalping, to reduce the foreign material to 5 to 15 percent of the volume by the contracting seed company. The goal is to reduce the level of foreign matter to 3 percent or less to obtain 97 percent pure seed and to remove nonmarketable seeds prior to sizing. After scalping, the seed is further cleaned to bring the product to 98 percent pure seed and to ensure desirable seed size. Growers will be docked if extra conditioning is needed, such as clean out fees for low germination or impurities such as weed seeds. All seed must be sampled and tested after conditioning and lots are certified as required by export markets. For certification, all samples drawn for seed analyses must be drawn by an Association of American Seed Control Officials accredited seed sampler. Currently, CCIA trains AASCO accredited seed samplers. Every 3 years, all accredited samplers must renew their accreditation by attending training seminars that CCIA conducts. If a company or conditioner does not have an accredited sampler on staff, an accredited individual from the agricultural commissioner’s office will draw samples for them. After the seed company submits information on the seed lot and seed analysis to CCIA, CCIA reviews it. When standards are met, CCIA issues a seed inspection report, after which, companies may request tags. If the seed is intended for shipment to a country that is a part of the OECD Seed Schemes , a company must also apply through CCIA for an OECD certificate. The tags, OECD certificate, and phytosanitary certificate must accompany the shipment.