MAJOR DISEASES

     Oilseed rape is a winter annual and belongs to the crucifereae (mustard) family. The term canola refers to the varieties of rapeseed that produce high quality, edible oils and low glucosinolate seed meals. Most canola quality varieties belong to either Brassica rapa or Brassica napus species. These are closely related to turnips, collards, mustard, cabbage, cauliflower and broccoli. The word rape in rapeseed actually comes from the Latin word rapum which means turnip.
     The seed meal produced after oil extraction is approximately 37 percent protein and is used as a high protein feed for poultry or livestock. Extensive breeding programs are under way to improve the quality of oil and protein, especially for the edible types. The current canola quality varieties are improved types with very low levels of erucic acid and glucosinolates.
     Canola planted commercially in Georgia in 1993, 1994 and 1995 had yields ranging between 35 to 40 bu/A. Their profits exceeded other traditional crops such as wheat.
     As with most crops, pest problems can reduce profits. Canola yields can be reduced by numerous pests including insects, weeds and diseases. Knowledge of important canola diseases enable growers to develop suitable management practices for future plantings.
     Many fungi attack this crop, but blackleg (Leptosphaeria maculans) and Sclerotinia Stem Rot (Sclerotinia sclerotiorum), are two major Georgia diseases.

     Blackleg, caused by the fungus Leptosphaeria maculans, is the most serious canola disease in most of the world's production areas. The spores that cause infections arise from infested and infected seed and crop litter. Seed is an important means by which blackleg is introduced into new production areas. Blackleg was first reported in Georgia on canola during the 1992-1993 season and has spread to almost all counties producing canola. It has caused significant losses in some fields planted to susceptible varieties.
     The fungus causing this disease can arrive in a field as a seed infection or by airborne spores (Figure 1). The fungus can form spores on debris or stubble from the previous growing season. These spores can be transported to neighboring fields up to 5 miles away. Once introduced, litter and several weed species can harbor the pathogen for the next growing season.

Leaf infections initially appear as round to irregular shaped lesions (Figures 2 and 3). They are white to tan in the center and become papery often breaking up as they age. They are almost always dotted to some extent with small black bodies called pycnidia. Other leaf spots may appear similar to blackleg lesions, but lack the definite border and do not develop the black dotted appearance characteristic of blackleg.      Disease losses occur when stem cankers develop at the crown of adult plants, weakening this area and causing the infected plants to fall over. Infection can occur at any time throughout the season. Most adult plant cankers are a result of fall infection occurring between the cotyledon and 6th to 8th leaf stage. Adult plant cankers are most often due to systemic movement of the fungus from early (fall) leaf lesions into the plant crown. Leaf lesion numbers or ratings are typically not an accurate predicter of adult plant cankers and loss (Figures 4, 5). Blackleg Control       The best way to control blackleg is by using resistant varieties. The other recommended control practices are, however, important and are aimed at keeping disease pressure as low as possible. A canola blackleg management program is outlined below:      Resistant Varieties - Using resistant varieties is the most effective and economic means of controlling blackleg in canola. Consult the latest Georgia Canola Production Guide for recommendations on blackleg resistant cultivars best suited to your production area.      Litter Destruction - As soon as possible after harvest, deep plow to bury all crop residue. Litter from ALL varieties, resistant or not, can be colonized by the fungus and serve as a source of infection in the next crop. Do not no-till susceptible summer crops behind canola, in fields where blackleg was observed. Once the canola litter is buried, cultivate summer crops as little as possible to avoid bringing the litter back to the surface of the soil.      Crop Rotation - Do not plant canola behind canola. Keep canola out of fields with old canola litter. Keep canola crops as far apart in distance (3 to 5 miles) and time as possible.      Seed Treatment - Use seed treated with Benlate or other labelled fungicides. Contaminated seed is the major means by which blackleg is introduced into new production areas.

Figure 2. Blackleg Symptoms on Canola Foliage. Figure 3. Black Pycnidia Within the Lesion. Figure 4. Stem Symptoms at Base of Plant Responsible for Lodging (collapsed tissue at base of stem). Figure 5. Severe Lodging from Blackleg on Susceptible Canola Variety.

     Sclerotinia sclerotiorum is a soilborne fungus with a wide host range and this fungus is common in almost all fields (Figure 6).

Figure 6. Life Cycle of Sclerotinia Stem Rot Fungus.

Infections appear as soft, water soaked areas on the stem. As lesions expand up and down the stem, they become gray with faint concentric markings. In wet weather or deep in the canopy where drying is slow, the lesions may be covered by white fungus growth (Figure 7). As lesions age, the stems may appear soft, often mushy when wet, and somewhat shredded under dry conditions. Infected plants eventually die and take on a distinctive bleached appearance. Infected plants usually break off, lodge and can collapse before harvest. Stems of infected plants may contain one to several hard, somewhat irregular-shaped black bodies (1/8-1/2 inch long) called sclerotia which are responsible for survival of the pathogen from one season to the next (Figure 8).

The fungus responsible for Sclerotinia stem rot causes two types of infection. The most typical means of infection occurs when moisture conditions and temperatures are favorable for germination of sclerotia to produce more or less golf tee-like structures called apothecia (Figure 9). Spores produced in the cup portion of the fungus are scattered by air currents through the canopy. Flowering often occurs, particularly in north Georgia, during the time the spores are formed in the apothecia. Infections can occur on any of the plant parts including flowers or petals and infections of Sclerotinia stem rot are, therefore, most severe when apothecia development coincides with flowering. The spores cannot directly infect a healthy stem, but require a food source such as a senescing petal or flower.      The second type of infection occurs when the fungus initiates growth in the soil from overwinter structures (sclerotia) and comes in direct contact with decaying leaf tissue (Figure 8). The fungus moves up the decaying leaf and into the crown. This type of infection usually occurs when plants are in the rosette stage, but later infections to stems can result.      Canadian scientists find that to start the disease cycle, sclerotia must be in the top 1 to 1½ inches of soil. The temperature must be cool (50 to 65°F) and must have continuous moisture 10 days or more. In Canada, most losses occur from infection arising from spores produced in apothecia during the bloom period. In Georgia, Sclerotinia stem rot varies in severity each year. In northwest Georgia, Sclerotinia stem rot is greater due to longer periods of cool soils.      Apothecia were observed in north Georgia during canola bloom in 1990 through 1995. Losses near Rome, Georgia have approached 30 to 40 percent of stand during bloom, but were much lower in 1995. Yield losses in these fields are unknown. Based on the Canadian loss estimate model, yield losses of 12 to 20 percent would be expected.

Figure 7. Stem Symptoms of Sclerotinia Stem Rot on Canola. Figure 8. Sclerotia (small black structures) of Sclerotinia Rot on Canola. Figure 9. Apothecia, Golf Tec-Like Structures, of Sclerotinia Stem Rot.

     One of the problems in producing canola in Georgia has been getting healthy stand establishment. The importance of seedling disease in production of canola in the state is unclear. Rhizoctonia solani AG-4 is presently the only anastomosis group known to kill the canola seedlings in the state.
     Rhizoctonia solani AG-4 can cause seed rot and kill seedlings up to the fourth leaf stage. Losses are not generally serious, but usually occur when seeds are planted in excessively cool and wet soils. Planting canola during the recommended period will coincide with optimum soil temperatures most years.

     Altemaria leaf spot is often referred to as "dark leaf spot" and will occur wherever canola is grown. Altemaria leaf spot is only of minor concern in the canola production areas of Georgia. The most common symptoms are found on the leaves, pods and stems. Foliar lesions can vary in size but start as dark spots and become gray to black, depending on the weather. The lesions become circular or irregularly large as they expand. As the crop matures, heavily-infected pods take on a gray sooty appearance. The infections result in premature ripening which causes poor quality seed and excessive shattering.
     The fungus overwinters in infested crop residue, on or in seed, on other winter crucifers (cabbage, collards, turnips) and on certain weeds such as wild mustard. If seed are infected, they will either rot in the soil after planting or develop into infected seedlings. Warrn humid conditions favor disease development and can potentially be very damaging in wet springs.

Altenaria Leaf Spot Control

          1. Plant clean seed to reduce seedborne infections of the fungus.
          2. Allow at least four years between canola crops in a field.
          3. Remove susceptible weeds and volunteers.
          4. Dispose of previous years stubble.
          5. Do not plant canola behind other crucifers.

     The fungus occurs on damaged tissues resulting from insects or fertilizer injury. When infected canola tissue comes in contact with disease free tissue, the fungus can invade causing a gray fuzzy growth particularly during humid and wet weather.

     Powdery mildew can be recognized by the white cotton growth, on above-ground parts of the plant. In spring, moderate temperatures and high relative humidity, increase the incidence of the fungus. High nitrogen levels and closed canopy are ideal for development of powdery mildew. In south Georgia, the disease has caused significant yield losses under limited production conditions, especially in 1990-1991.

     The symptoms are light (approaching white) papery lesions with irregular borders. There may be a diffuse, sparse black growth of the fungus in the lesions. White leaf spot should not be confused with leaf lesions caused by the blackleg fungus which have more regular borders and are dotted with black fungus bodies erupting through the leaf tissue.

     Black rot is a common seedborne disease of cabbage and other crucifers. The disease causes yellow wedge shaped spots extending in from leaf margins. Black rot has not caused major losses in canola production fields. Black rot can remain active in non-decomposed plant litter for up to two years. Do not plant cabbage or canola within three years of growing either crop in the same field.

     Root-knot nematode (Meloidogyne spp.) or RKN is the only species of nematodes thus far found to adversely affect production of canola in Georgia. Results from preliminary studies in Georgia and surrounding states, however, are inconclusive. Researchers believe that when canola is actively growing, nematodes are generally dormant unless soil temperatures are unseasonably warm. Further investigations must be conducted before any conclusive statements can be made concerning nematode effects on canola. Refer to the most recent Georgia Canola Production Guide for current updated information.

The Cooperative Extension Service, The University of Georgia College of Agricultural and Environmental Sciences offers educational programs, assistance and materials to all people without regard to race, color or national origin.