The Sclerotinia diseases are some of the most important diseases of sunflower in the Northern Great Plains. Three diseases are recognized in the field: Sclerotinia wilt, middle stalk rot, and head rot. Wilt is distinct because it begins as a root rot, whereas Sclerotinia head rot and middle stalk rot are above-ground diseases caused by airborne spores. All three diseases are caused by the same organism, Sclerotinia sclerotiorum, a very destructive fungus which is often called the "white mold" fungus.
Sclerotinia sclerotiorum is a serious problem not only in areas where sunflower and dry bean have been cultivated for many years, but also in areas where irrigated susceptible crops are grown. As production of sunflower in North Dakota moved west, there was an increase in the incidence and severity of Sclerotinia diseases in areas with no previous history of the pathogen. This fungus can be a serious problem in the drier sunflower production areas.
Sclerotinia sclerotiorum attacks approximately 374 species of broadleaf plants. The row crops commonly damaged in the Northern Great Plains are sunflower and dry bean; however, many other crops are susceptible, such as soybean, buckwheat, flax, lentils, pea, potato, mustard, crambe, rapeseed or canola, Jerusalem artichoke and safflower. Not all of these other crops are as susceptible to S. sclerotiorum as sunflower and some are rarely damaged. Many broadleaf weeds
such as marsh elder, common lambsquarters, redroot pigweed, Canada thistle and wild mustard also are hosts.
Description: Characteristic symptoms include sudden wilting of leaves, root rot and a basal stem canker (Figure 61 JPEG; 21.6Kb). Wilting plants are often first observed just prior to flowering, but about 60 to 70 percent of the wilted plants appear after flowering and are commonly found adjacent to each other. The time required from incipient wilt to complete wilting may be four to seven days.
A tan, greyish or green-brown canker forms at the base of the plant and eventually girdles the stem (Figure 62 JPEG; 28.5Kb). As decay progresses, the stalk becomes bleached with a shredded appearance and the pith is decayed. Plants lodge easily during high winds. Inside and often outside at the base of the stem hard, black resting bodies of the fungus called sclerotia [about 0.12 to 0.25 inch (3 to 6 mm) in diameter] can be found (Figure 63 JPEG; 22.7Kb). The presence of sclerotia provides a positive identification of the disease. Decay of lateral roots and tap roots is obvious. During wet weather, white mycelium (mold) often develops at the base of the stem, hence the name "white mold."
Disease Cycle: Sclerotinia sclerotiorum overwinters as sclerotia in the soil or in plant debris. When sunflower roots come in contact with sclerotia, the sclerotia germinate, infect and decay the roots, the fungus grows up into the stem, and the plant wilts and dies. Contact between roots of adjacent plants within rows allows the fungus to spread from plant to plant. The fungus generally does not move between rows. Sunflower is the only crop S. sclerotiorum consistently infects through the roots. Other susceptible crops are infected mainly by spores on above-ground parts of the plant.
Sclerotia are formed in the decayed stem pith and on the roots as the plant dies. These sclerotia are then returned into the soil during tillage operations and serve as sources of inoculum for the next susceptible crop. Sclerotia can be spread from field to field through wind-blown soil, moving surface water, on farm implements and rarely as contaminants in commercial seed.
Sclerotia survive in the soil and fields can remain infested for many years. The higher the inoculum density (i.e., the number of sclerotia in the soil), the longer the period a field will remain infested. Populations of sclerotia begin to decline when fields are planted with non-hosts. Although the effect of tillage practices on sclerotia survival is little understood, preliminary research suggests minimum till will enhance the degradation of sclerotia.
The most important factor affecting incidence of wilt is inoculum density. Research in North Dakota has shown that an inoculum density of 0.1 sclerotia per liter (about one quart) of tillage layer soil can result in about 10 percent wilted plants. An inoculum density of 1.0 sclerotia per liter of soil would result in about 60 to 70 percent wilted plants in the standard hybrid 894. In practical terms, this means that low levels of the fungus in the soil will cause substantial sunflower losses. Soil moisture and temperature during the growing season are not critical factors affecting wilt incidence. Plant population in the range between 15 to 30 thousand plants per acre (37-74 thousand plants per hectare) on 30- or 36-inch (76- or 91-cm) rows is not a factor affecting disease incidence (percentage of plants that are diseased), although solid-seeding should be avoided. Lodging of wilted plants, however, increases at high plant populations.
Damage: Wilt is the most important of the three diseases caused by Sclerotinia. Wilt occurs whenever sunflower is planted on Sclerotinia-infested soil and can cause severe yield loss. For example, in two fields in North Dakota with 80 percent and 60 percent incidence of wilt, yield was reduced by 79 percent and 50 percent, respectively, in the hybrid 894. Infected plants usually die rapidly. Surviving plants may or may not produce seed, depending on when infection occurs. The heads on wilted plants generally are smaller than those on healthy plants and seed weights are lower. On the average, infected plants yield less than 50 percent of healthy plants. Equally important, however, is that Sclerotinia wilt leads to increased levels of sclerotia in the soil, which can result in the removal of fields from sunflower production for many years. Sclerotinia diseases, therefore, affect future production and economic gain from sunflower and other susceptible crops and disrupt rotation schedules.
Sclerotinia Middle Stalk Rot and Head Rot
Description: Middle stalk rot is usually first observed at the middle to mid-upper portion of the stalk during flowering and continues until maturity. It begins as a tan to gray water-soaked lesion which eventually girdles and decays the stalk (Figure 64 JPEG; 25.7Kb). The stalk usually bends over at the point of decay and the tissues above the canker die. During wet weather, a dense white mycelium and some sclerotia often will be produced both inside and outside the stalk. The affected tissues become bleached and will have a shredded appearance.
The first symptoms of head rot usually are the appearance of water-soaked spots or bleached areas on receptacles (the fleshy back of the head). The fungus can decay the entire receptacle and the seed layer falls away leaving only a bleached, shredded skeleton interspersed with large sclerotia (Figure 65 JPEG; 28.1Kb). These bleached, skeletonized heads are very obvious in the field, even from a distance. During combining, infected heads often shatter and any remaining seeds are lost. Usually, the seeds are not decayed but many are empty. The large sclerotia in the heads may be 0.5 inch (12 mm) or greater in diameter and many are harvested along with the seed (Figure 66 JPEG; 25.6Kb). Large sclerotia mixed in with seed confirms that a field contained head rot.
Disease Cycle: If soil moisture is high for seven to 14 days, sclerotia in the upper several inches of soil can germinate to form small mushrooms called apothecia. These apothecia produce ascospores for a week or more if soil moisture remains high. The ascospores are ejected into the air, carried by the wind, and may land on sunflower or other susceptible plants. Apothecia can be found between June and September but usually are not observed until after the crop canopy has completely covered the rows. Apothecia are formed under canopies of many crops, including small grains, corn, potato, dry bean, soybean and sunflower (Figure 67 JPEG; 20.1Kb).
Ascospores require a film of water and a food base such as dead or senescing plant tissue to germinate and infect, and are also known to infect through wounds. On the stem, the ascospores apparently come in contact with a food base (such as dead pollen) and water that accumulates in the stem-leaf axil and infection occurs. Infection may also occur on a damaged leaf, and the fungus will grow down the petiole into the stem. Infection of sunflower by ascospores is similar to the process by which S. sclerotiorum infects dry beans, soybean and other susceptible crops. Ascospores infect the receptacle of the sunflower head via the developing florets and decay the entire head. The pathogen can infect the seed and survive as mycelium (thread-like strands of the fungus) in the seed coat, but evidence suggests that infected seed is not an important means of spreading of the fungus.
Damage: Head rot and middle stem rot occur sporadically and only following long periods of wet weather. Usually they are not as serious as wilt in the Northern Great Plains, although they can cause yield losses. Rotted, intact heads may yield up to one-third less than healthy heads and are very susceptible to shattering, so additional seed is lost during combining. Head rot also causes a decrease in oil content and an increase in free fatty acid content. The sclerotia that form in diseased stalks and heads are returned into the soil at harvest and can cause Sclerotinia diseases in sunflower or other crops in following years, magnifying the damage. Many sunflower fields with no history of Sclerotinia became infested due to ascospore infection followed by a return of sclerotia to the soil.
Management of Sclerotinia Diseases: Because occurrence of middle stem rot and head rot is sporadic, disease control is usually aimed at control of wilt. The most important tools for managing the Sclerotinia diseases of sunflower are planting in non-infested soil and preventing buildup of sclerotia in soils. Prevention is done principally through monitoring of fields for Sclerotinia diseases and crop rotation. No totally resistant hybrids are currently available. However, commercial seed companies are in the process of developing hybrids that are moderately to highly resistant to Sclerotinia wilt. There is no chemical registered for control of wilt and none that is economic. There are no fungicides currently registered in North Dakota for control of ascospore infection of sunflower. The following is a summary of disease management recommendations:
- Sunflower should not be planted on land already infested with sclerotia. Fields that have been planted to susceptible crops such as dry bean or soybean could be infested, especially if in an area where Sclerotinia is common.
- Planting certified seed minimizes the danger of introducing sclerotia into fields that are free of Sclerotinia. Avoid solid seeding and high plant populations.
- Fields of susceptible crops should be monitored for disease incidence. Sunflower fields should be scouted about four weeks after flowering to assess incidence of wilt. A later date is even better, but as dry-down proceeds, it becomes harder to evaluate infected plants. A final scouting should occur prior to harvest to assess incidence of middle stalk rot and head rot. In other susceptible crops such as dry bean, it may be necessary to carefully search beneath the canopy to see sclerotia being formed. Accurate records of disease incidence and crop rotations are necessary for managing this pathogen.
- Rotations to a nonsusceptible crop such as small grains, corn or sorghum are necessary when disease appears (see section on rotations). Crop rotation is the most important management procedure. The rotation interval will depend upon disease incidence. A three to five-year rotation may be necessary with low disease incidence (less than 10 percent), while six to eight years or longer might be needed at higher disease incidence. A dryland field with 10 percent wilted sunflower plants might require a four to five-year rotation to nonsusceptible crops to reduce the incidence of wilt to about 5 percent. The incidence of wilt should not be permitted to exceed 1 to 2 percent before starting a rotation to non-host crops. A low incidence of wilt increases substantially after several years of continuous sunflower and long rotations are then needed when there is a high level of sclerotia. Broadleaf weeds should be controlled.
- If sunflower is to be planted on Sclerotinia-infested soils, choose the least susceptible commercial hybrids available.
- Sunflower should not be planted adjacent to a field infested the previous year because this may serve as a source of ascospores for head and middle stalk rots.
SOURCE: NDSU Extension Service Circular PP-840 * Goes to a non-federal site