Seed companies frequently treat seeds with an insecticide and several fungicides to control seedling pests and diseases to ensure good stand establishment. A mixture of seed treatments includes the insecticide Cruiser 5FS and the fungicides Dynasty , Bion , or Plenaris for downy mildew, Maxim for Fusarium, and Apron XL for Rhizoctonia, Pythium and Phytophthora.The pustules turn black with the onset of cool temperatures and develop into overwintering spores. Rust completes its entire life cycle on sunflower and does not require an alternate host like cereal rusts. Rust overwinters on plant debris and infects volunteer seedlings or wild sunflowers, and spores are spread by wind to other sunflower plants. Rust multiplies rapidly under favorable conditions of warm temperatures and either rain or dew. Thus, even in dry years, if night temperatures are low enough to promote dew formation on leaves, this minimal amount of leaf wetness will be sufficient to initiate rust infection. Excessive rates of nitrogen fertilization may result in excessive foliage, which increases humidity within the canopy and favors rust development. Destroy volunteer plants and wild annual sunflower growing near commercial fields as early in the spring as possible to reduce sources of inoculum. In seed production fields, vertical farming supplies rust resistance is generally carried by the male pollen-producing line. Since the male plants are destroyed after pollination, the concern is for the seed-bearing female plants.
Several fungicides effective against P. helianthi with different modes of action include azoxystrobin , Orius 3.6F and Luna . From a practical perspective, if any rust is found in a field, it will fail phytosanitary inspection; thus, fungicide applications that do not achieve 100% control would be unwarranted, leaving early-season roguing the most practical alternative for rust control. In oilseed-producing regions, the most effective way to avoid loss from rust is by planting rust-resistant hybrids. Seed companies are continually testing their hybrids in different locations to determine rust resistance against the various races in different regions. Downy Mildew Downy mildew, caused by the pathogen Plasmopara halstedii, produces white cottony masses primarily on the lower leaf surface during periods of high humidity or dew . Unlike downy mildew in other crops, the fungus initiates a systemic infection via root colonization of young seedlings and is systemic within the plant. Symptoms in seedlings include dwarfing and yellowing of the leaves that can cause plant death. If plants are not killed in the seedling stage, heads on the dwarfed plants are borne horizontally, and the achenes are often nonviable or blanks . Sunflower plants are susceptible to infection in the seedling stage, before the roots are 2 inches long. This period may range up to a maximum of 2 to 3 weeks, depending on soil temperature and moisture. Cool, water-saturated soil during this period greatly favors infection. In addition to systemic infection, airborne spores may cause local lesions on leaves, which seldom lead to systemic infection.
Downy mildew is spread by soil, wind, and seeds. The fungus can persist as spores in the soil for 5 to 10 years. This pathogen does not infect any species other than those in the sunflower family,including all wild Helianthus species and Jerusalem artichoke . Control volunteer sunflower or wild annual sunflowers that may serve as reservoirs for the fungus. Systemic downy mildew infections cannot be controlled or eradicated by foliar fungicide sprays. The best protection against downy mildew is by planting genetically resistant hybrids or parental lines, coupled with fungicide seed treatments. In seed production fields, downy mildew resistance will generally be only on one parent line, but an effective seed treatment will be on all planting seeds. While genetic resistance is effective, the downy mildew population in sunflower is ever-changing, and there are currently more than three dozen races found in the United States. Collar Rot Collar rot, caused by Phytophthora spp., is a sunflower disease observed in California and not in the Midwest. This disease is favored by warm, moist soil, and is thus observed in over irrigated sections of fields in midsummer. Symptoms of the disease include a black, girdling lesion at the soil line, most often seen at the ends of rows nearest the irrigation ditch for furrow irrigated crops . No hybrids or parental lines are known to have resistance, and no fungicides are registered for control of this disease. The best management practice is to avoid soil compaction which could stress plants and carefully irrigate fields without excess water.
Charcoal Rot Charcoal rot, caused by the fungus Macrophomina phaseolina, generally appears after flowering but seedling blights have been reported. Symptoms on stalks appear as silver-gray lesions near the soil line, which eventually decay the stem and tap root, leaving a shredded appearance and hollow stems that lodge easily. Numerous tiny black fungal bodies resembling pepper grains will form on the inside and outside of the stalk that are barely visible, but easily seen with a hand lens . Another unique characteristic of charcoal rot is the compressing of pith tissue in the stems into horizontal layers, like a stack of coins. Infected plants will show undersized heads and poor seed fill. The primary source of inoculum is microsclerotia in the soil, but charcoal rot can also be seedborne. Plants are infected through roots only after being predisposed by drought stress. The fungus is favored by high temperatures. Good water management to avoid stressing plants is important in managing this disease, particularly as the crop approaches the flowering stage. Crop rotation to nonhost crops, such as small grains, can also help reduce the disease potential. The charcoal rot fungus has a wide host range, including beans, corn, and sorghum. Charcoal rot persists in the soil for 2 to 3 years. There are no registered fungicides to control this disease. Head Rot Head rot, caused by Rhizopus spp. fungi, is a common occurrence on sunflowers damaged by insects , birds, or hail. Rhizopus spp. rot the soft tissues of the head, turning it brown and mushy . Once the head is fully infected, black, pepper grain–sized spores of Rhizopus may be seen. Eventually, the head dries up and becomes hard and “mummified.” The susceptibility of heads increase from the bud stage up to the full bloom and seed development stages. Disease development is most rapid in warm, humid weather. Insect control is the main management practice to minimize Rhizopus head rot. No resistant hybrids or registered fungicides exist for Rhizopus head rot control. White Mold White mold, caused by Sclerotinia sclerotiorum, is likely the most serious sunflower fungal pathogen in California, the Midwest, weed rack and around the world. It is unique in that it may cause three different symptoms. It primarily causes a basal root rot and ensuing wilt , which is caused when the soilborne sclerotia germinate and infect root systems. Symptoms, which generally appear close to bloom, start as wilted leaves that never recover, and a brown, girdling lesion at the soil line. With continuous moisture, this lesion may be covered with a white fungal mass , leading to the common name of white mold. If the stalk is split open it may be hollow in the lower part of the stem and will also be filled with tiny, black, irregularly shaped spores, ranging in length from a ¼ to over 1 inch. The other two disease phases caused by white mold are midstalk rot and head rot, both of which are initiated by airborne fungal spores from the soil. When the spores land on injured leaf tissue , the fungus colonizes the leaf , then progresses down the stem, eventually causing a tan, girdling lesion at the leaf base.
The stalk becomes hollow at this point and may lodge. When the spores land on a head, they colonize dead pollen and penetrate the head. For both midstalk rot and head rot to be initiated, the spores require a continuous wet period of more than 8 hours. While summer rains are nonexistent in California’s sunflower-growing regions, dew would be sufficient to allow spore germination. The fungus will eventually completely destroy the head, leaving only the vascular strands, giving the appearance of a broom. If the disease does not completely destroy the head, the resultant seeds will be smaller. Numerous fungal spores will form in the head, many of which are the same shape and size the sunflower seeds, resulting in dockage or complete refusal by the seed company. These sclerotia spores are also dispersed by a combine at harvest, resulting in a field contaminated with the fungus for several years. White mold is best prevented by monitoring and controlling the disease in crop rotations to avoid the buildup of inoculum in the field. No fungicides are currently registered in California for white mold control in sunflower, but some are registered in other crops, such as Endura in beans or Pristine in alfalfa. The fungus infects nearly every broad leaf crop, and the sclerotia can persist in the soil for up to a decade. While some hybrids have some level of resistance, none currently could be classified as fully resistant. A related fungus, S. minor, causes a similar basal stalk rot and wilt and is distinguished by its small, round black sclerotia, compared with the much larger and irregularly shaped S. sclerotiorum sclerotia. S. minor does not cause midstalk or head rot. Powdery Mildew Powdery mildew, caused by the fungus Golovinomyces cichoracearum , produces a dull white to gray coating mainly on the upper surface of lower leaves, causing them to dry up and senesce prematurely . Powdery mildew is seldom seen until late in the growing season, since senescing leaves are most susceptible to infection. Sunflower cultivars differ widely in their susceptibility to powdery mildew. The disease generally occurs late enough in the season that control measures are not needed. Powdery mildew is not currently classified as a disease of quarantine significance by any importing country.Verticillium Wilt Verticillium wilt, caused by the fungus V. dahliae, first appears as yellowing on older leaves, causing interveinal chlorosis, with leaves turning progressively brown . The foliar symptoms progress up the plant until all leaves are affected. A light tan to brown discoloration develops in the vascular tissue of the stalk, especially near the base of the plant . Symptoms are most noticeable during later stages of plant development, with the fungus causing the pith in stalks to become blackened and shriveled . The fungus survives as microsclerotia in the soil. Once established in a field, it persists indefinitely and can cause disease whenever a susceptible host is planted, which includes a large number of crops as well as weeds. The disease is favored by cool soil and air temperatures. To manage Verticillium wilt, rotate to nonsusceptible crops, such as small grains and corn, and Verticillium resistant crops to help reduce the soil inoculum. Field sanitation, especially washing field equipment, is important to prevent movement of infected soil to clean fields. Southern Blight Southern blight, caused by Sclerotium rolfsii, is a soilborne fungal pathogen that initially causes a yellowing of the foliage with a slight darkening of the stem just above the soil line. Lesions rapidly develop, girdling the stem, and causing sudden and permanent wilt of the plant . The fungus colonizes the root system and lower stem, eventually destroying the pith, much like the damage from white mold. White mats of fungi develop on the stem and nearby soil when there is sufficient soil moisture. In a few days, tan to brown spherical sclerotia may appear on the fungalmat. If present, these are a good diagnostic feature of this disease. Southern blight is usually a minor disease of sunflower and other crops in California. However, temperatures above 85°F together with high moisture favor the disease. The fungus survives for a long time in the soil as sclerotia. Rotations are not considered effective since this pathogen has over 500 hosts. However, it is thought that rotations to corn or small grains for at least 2 years may reduce soil inoculum; studies are underway to determine whether these rotations are effective. Discing and deep burying crop stubble helps destroy sclerotia.