Sunflower Pest Management (cont.)

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

Diseases (cont.)

Head Rots

Description: Several head rots occur on sunflower in the Great Plains, caused by Rhizopus, Botrytis, Erwinia, and Sclerotinia.

Rhizopus head rot, caused by Rhizopus arrhizus Fischer and R. stolonifera (Ehrenb. ex Fr.) Vuill., is a sporadic disease in the Great Plains but has caused extensive damage in some areas of Texas, Kansas, and California (Figure 73). Initial symptoms are brown watery spots on the receptacle. These fungi rot the soft tissues of the head turning it brown and mushy. A thread-like, whitish fungus mycelium develops on and within the receptacle. Tiny black pinhead size fruiting structures form on the mycelium. Rhizopus head rot losses in the Great Plains are usually low, but seed yields in Texas have been reduced as much as 20 percent and oil percentages reduced as much as 45 percent. Free fatty acid content may increase by 20 percent.

Rhizopus enters the head through wounds caused by hail, birds and insects and has been associated with head moth and midge damage. The susceptibility of heads increases from the bud stage up to the full bloom and ripening stages. Disease development is most rapid in warm humid weather. Insect control may help minimize damage although this is not suggested as a means of disease management without first ascertaining whether insect control is necessary. Rhizopus head rot can easily be misidentified as Botrytis head rot (Botrytis cinerea Pers.).

Botrytis head rot seldom causes serious damage in the Great Plains. Symptoms usually occur during head maturation as brown spots on the back of the head. These spots are eventually covered by gray powdery spores of the fungus, giving the head a "fuzzy" appearance. These spores generally form on the surface tissues and not inside the tissues as is common with Rhizopus. In wet weather, the spots spread, invade all through the tissues, and the head becomes a rotten spongy mass. The fungus infects the head during cool, wet weather and requires organic debris such as flower parts or senescing tissue to initiate growth. Late attacks start from the senescent petals and head bracts and may be serious when there is a wet fall and late harvesting. No specific management practices are recommended in the Great Plains.

Bacterial head rot caused by Erwinia carotovora is rare in the Great Plains. This disease is characterized by a slimy, wet brownish rot of the head with no fungus growth or spores in the tissues. Often such heads have a putrid odor. The bacteria enter the head through wounds caused by birds, hail and insects.

Sclerotinia head rot was described under the section on Sclerotinia diseases. Sclerotinia head rot can be readily differentiated from other head rots by the large sclerotia produced in the receptacle and broom-like appearance when the rotted heads disintegrate.

Powdery Mildew

Powdery mildew, caused by the fungus Erysiphe cichoracearum DC, can be found in most fields after full bloom. Powdery mildew is seldom a problem in the Northern Great Plains but can be serious in the sunflower-producing areas of the Southern Plains.

The disease appears in the form of white (later gray-tan) mildewy areas, primarily on the leaves, but also on all aerial parts of the plant under heavy infections (Figure 74). This superficial mycelium (fungal growth) may enlarge and merge until most of the plant surface is covered. As the season progresses, the mildewed leaves take on a dusty, powdery appearance. This powder is easily rubbed off. Small black dots, the sexual fruiting structures of the fungus, may appear scattered within the mildewed areas late in the season. Severely infected areas become permanently yellow and may dry up. Normally the lower leaves are more heavily infected than the upper leaves.

Powdery mildew generally occurs late enough in the season that control measures are not needed. Sunflower cultivars differ widely in reaction to powdery mildew.

Apical Chlorosis

Apical chlorosis is the only bacterial disease of sunflower that has been of any significance in the Northern Great Plains. The causal organism is Pseudomonas syringae pv. tagetis (Hellmers) Young, Dye and Wilkie. Apical chlorosis is striking and seldom goes unnoticed but is seldom significantly reduces yields. The major symptom of the disease is the extreme bleaching or chlorosis of the upper leaves (Figure 75). The disease occurs only during the vegetative growth stage and is most severe in young seedlings during cold weather and in water-logged soils. Apical chlorosis may be distinguished from iron chlorosis or nitrogen deficiency by the complete lack of green pigment and the uniformity of the chlorosis. With mineral deficiencies, the veins characteristically remain green. In addition, the white leaves affected by apical chlorosis will never "regreen" while those due to mineral deficiencies will.

Plants affected by apical chlorosis will usually produce new green leaves in several weeks with little discernible effect other than striking white leaves in the middle of the plant. However, infected young seedlings will die when stressed by cool temperatures and water-logged soils.

No hybrids are completely immune to the disease. The only control recommendation at present is to follow a normal rotation to avoid increasing the population in the soil. Roguing infected plants in seed production fields will eliminate the disease.

Premature Ripening

Premature ripening is a broad term used to refer to a set of symptoms including thin, weak, blackened stems, lodging, smaller heads with empty or partially filled seeds and affected plants, often in patches. Seed yields, oil content and test weight are severely reduced in these plants. These symptoms are commonly observed in the upper Great Plains, and in some years extremely large acreages of sunflower production have been affected. The symptoms of premature ripening (also referred to as early dying) are caused by a number of pathogens, probably in association with specific environmental conditions and possibly specific cultivars. The factors associated with this type of disease development are not completely understood. Phoma black stem, Sclerotinia wilt, charcoal rot, Alternaria leaf blight and other diseases, either alone or in combination, can cause premature ripening (see description of these diseases). There may be other pathogens involved which have not been identified. Stresses caused by drought or insects such as stem weevils appear to contribute to disease development. Crop rotation of three to four years is the current disease management technique.

Nematode Diseases

Description: Nematodes are microscopic, nonsegmented worms that do serious damage to the roots of many crops. While many different types of nematodes have been found in sunflower fields, their economic impact on the plant is undocumented or highly variable.

Genera of nematodes that have been reported on sunflower in the Dakotas include: Heliocotylenchus, Tylenchrohynchus, Paratylenchus, Hoplolaimus, and Xiphinema; the first two genera are the most widely distributed in North Dakota, while Paratylenchus is the dominant nematode in South Dakota. All these nematodes are ectoparasites, meaning that they feed either on the root surface or burrow partially into the roots. Root-knot nematodes (Meloidogyne spp.), which are damaging pests of many crops, have been reported on sunflower in Florida and warm areas of other countries but have not been recorded on sunflower in the upper Great Plains states. Sunflower is not a host for the soybean cyst nematode (Heterodera glycines Ichinohe), making sunflower suitable for rotation with soybean where the cyst nematode is a problem.

Symptoms caused by nematodes are not distinctive and mimic those due to drought and nutrient deficiencies. In severe infestations, the foliage wilts and turns yellow and stunting may occur. The pattern of affected plants in the field may have little or no relationship with topography. Examination of roots is necessary to prove nematode damage. Identification of nematodes to genus requires their extraction from soil and roots, followed by microscopic examination.

High populations of nematodes have caused yield reductions in greenhouse studies. Applications of nematicides in field trials have produced variable yield responses. No nematicides are currently registered for use on sunflower and the potential cost return makes their use questionable. Tolerance to nematode damage appears related to the extensive root system of sunflower.

Diseases Caused by Viruses and Mycoplasmas

Several viruses have been reported on sunflower from other countries and the warmer regions of the United States (Florida), but there are no confirmed reports of viruses occurring on sunflower in the Northern Great Plains. Wild sunflower is a host of tobacco ring spot virus in the Rio Grande Valley and cucumber mosaic has been reported on sunflower in Maryland. Viruses reported on sunflower outside the United States include tobacco streak, tomato big bud, sunflower rugose mosaic, and tomato spotted wilt. Confirmation/identification of a virus as the causal agent is based on observation of the viral particles using an electron microscope, detailed chemical analysis of the viral components, and serological identification.

Aster yellows is a disease at first believed to be caused by a virus but which has since been identified as a mycoplasma. Mycoplasma are living cells, in contrast to viruses, and are intermediate in size between viruses and bacteria. The aster yellows mycoplasma is only transmitted by the aster leaf hopper (Macrosteles quadrilineatus Forbes) and occurs on a wide variety of plants. Symptoms on sunflower include yellowing of leaves and/or the head, which often occurs in sectors. A characteristic symptom is a wedge-shaped portion of the head which remains green and bears small leaves rather than floral parts (Figure 76).

Parasitic Weeds

Sunflower is heavily damaged in countries bordering the Mediterranean and in China by a parasitic weed called broomrape (Orobanche cumana Wallr.). Although several Orobanche species occur in the United States, O. cumana has not been reported. Native Orobanche species are not known to parasitize wild or cultivated sunflower. The Orobanche plant is an obligate parasite which obtains all of its food from its hosts by means of root contact. The plant itself consists of a single stem up to 15 inches (38 cm) tall, with sessile, alternate yellow leaves (the entire plant lacks chlorophyll) and off-white to yellow flowers borne in a spike (Figure 77). A sunflower plant may be parasitized by up to a hundred Orobanche plants, which severely stunt the sunflower and decrease seed yield or kill the host entirely. Orobanche seed are smaller than the finest grains of sand (greater than 200,000 per ounce) and may remain viable for up to 10 years. Strict quarantine measures guard against introducing it into the United States. The cold winters of the Northern Great Plains also serve as a deterrent to the establishment of O. cumana