Seed Treatment Protects Seed from Pathogenic Fungi

Seeds can spread plant diseases from one farm to another, from one state to another, and from a distant country to the other. Control of such diseases begins with the seed, it is easier and cheaper to eliminate a pathogen from a few pounds of seeds than to attempt to spray or dust entire fields of growing plants. Seed treatment protects seed from pathogenic fungi associated with the seed otherwise, spores of pathogenic fungi adhere to the seed. When the seed is planted, these pathogenic fungi begin to grow, invade the seed or seedling, and cause a seedling blight. Other fungi live in the soil and may cause seedling problems. Soil-borne Pythium, Aphanomyces and Rhizoctonia fungi can cause serious stand loss when the soil is moist or wet. Tachigaren seed pelleting is highly effective against Pythium at lower rates and Aphanomyces at higher rates. Tachigaren persists for only 3-4 weeks and will provide protection only for the emerging seedling; it does not provide protection against mid-season infection. Commercial seed treaters apply tachigaren to sugarbeet seeds. Tachigaren can be used at 20 to 30 grams per unit (100, 000) of seed on minimum buildup pelleted seed, or 45 to 90 grams per unit of seed on standard pelleted seed. Rates greater than 45 gram of tachigaren per unit of seed may cause phytotoxicity. Use rate of 20 to 30 grams of tachigaren is recommended on fields with light to medium disease pressure. Growers with medium disease pressure, however, should be cautioned that use of 20 or 30 gram rate may be inadequate when soil is warm after a heavy rainfall or when these conditions are prolonged within 3 weeks after planting. Use rate of 45 grams of tachigaren is recommended for fields with heavy disease pressure. For season-long management of Aphanomyces, the best approach is to apply Tachigaren to varieties with partial resistance to Aphanomyces. Early planting and good drainage may also help reduce early season losses from Aphanomyces seedling disease. An Aphanomyces soil test should be done to determine if the soil is infected with Aphanomyces, and the level of infection.

Tips for good Yield:

1.       Plant high quality disease-free seed. Professionally grown seed is worth the added cost.
2.       Rotations with a grass crop such as corn or grain sorghum are generally beneficial in reducing population densities of soilborne pathogens.
3.       Soils which tend to be poorly drained and/or cooled should be planted later in the season when conditions for soybean germination and growth are optimal.
4.       Consider resistant or tolerant varieties for fields where Phytophthora (destructive parasitic fungi causing brown rot in plants) rot is frequently a problem.
5.       Seed treatments may be beneficial when planting in cool, wet soils or when seed is of marginal quality.
6.       When soybean are grown for seed, inspect the crop for diseases and harvest in a timely manner
7.       Spacing plants adequately and orienting rows with the prevailing winds aids in increasing air flow

Rhizoctonia In severely infected fields, plant resistant varieties early, avoid “hilling” soil on sugarbeet crowns, increase the length of rotation, and rotate with non-host crops. Penthiopyrad (KabinaST) seed treatment will provide early season contro l . Quadris and Headline applied in-furrow will provide early season control. Rhizoctonia may also be controlled by applying Quadris or Proline in a 7 inch band just before infection occurs, or when the average soil temperature at the 4" depth is about 60 to 62 F. Fields with a history of severe disease may need a second post application in warm and wet conditions for season long control.

Rhizomania, a virus disease caused by beet necrotic yellow vein virus (BNYVV), is one of the most economically limiting diseases of sugar beets wherever it occurs worldwide. It was first reported in Italy in 1959 and is now widespread throughout Europe. In the United States, it was first reported in sugar beets in California in 1984. It was first identified in Nebraska in 1992 and now has been identified in every major sugar beet production region in the country. Rhizomania is characterized by stunted taproots with masses of hairy lateral roots giving them a bearded appearance. The root is often constricted and the vascular tissues become discolored. The leaves become fluorescent-yellow (with elongated petioles) in color, similar to nitrogen deficiency symptoms. Rhizomania may be managed by planting approved resistant varieties early in well drained fields on a 3-4 year rotation. Select high resistance varieties for areas with known history of severe Rhizomania.

Fusarium yellows is typically caused by the fungus, Fusarium oxysporum f. sp. betae. However, recent research suggests that a new Fusarium species is also responsible for Fusarium of sugarbeet. Fusarium yellows may cause seedling death or poor growth and even death of older plants. Symptoms first appear on older leaves as chlorosis (yellowing) between the larger veins. As the disease progresses, younger leaves also become chlorotic, and the older, symptomatic leaves become necrotic. Occasionally, only half a leaf is chlorotic or necrotic (a symptom more typical of Verticillium wilt, which also was recently identified on sugarbeet in this region). Entire leaves eventually die but remain attached to the plant and collapse in a heap around the crown. There are no external root symptoms associated with Fusarium yellows. A transverse section through the root shows a grayish brown vascular discoloration. Infection of mature plants may not cause death, but the disease causes significant reduction in root yield and recoverable sucrose. In storage, quality of infected roots may deteriorate more rapidly compared to non-infected roots. The disease is favored by high soil temperatures.

Fields that are waterlogged, or with poor soil structure provide favorable conditions for infection. Crop rotation may reduce inoculum buildup in the soil but this practice is unreliable because F. oxysporum f. sp. betae has a wide host range and chlamydospores survive for many years. Use approved fusarium resistant varieties to manage this disease. Rotating crops is a good option to avoid disease build-up in the soil. Additionally, seed growers must use care in harvesting and cleaning seed to avoid post-harvest infection.


Reference : ndsu.edu
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