White Spot (fungus – Cercosporella brassicae): Lesions caused by Cercosporella varies from gray to brown in color. The center of the spots are light colored with slightly darkened margins (See Photo). Severely infected leaves turn yellow and drop. The fungus is seedborne and overwinters on volunteer plants. Spores are spread by splashing rain and airborne spores. The disease is most damaging when daily temperatures range between 55oF and 65oF and during periods of high humidity. Protective type fungicides should be applied on a weekly basis beginning when true leaves are formed if weather conditions are favorable for disease spread.[Table 2] Cultural practices that reduce leaf wetness and inoculum levels will aid in disease control. [Table 5]
White Rust (fungus – Albugo candida): The underside of infected leaves are characterized by white blister-like pustules (See Photo). The upper side will have faint yellow spot opposite the white lesion on the underside of the leaf (See Photo). Cruciferous weeds such as wild mustard and shepherd’s purse are other hosts for the fungus and are a source of inoculum for mustard and turnips growing nearby. The fungus is spread by windblown spores and overwinters in crop refuse. Disease development is most rapid at 68oF and during periods of light rain or dew. Control of the disease is based on cultural practices [Table 5]. Currently no fungicides are approved by EPA for the control of this fungus on mustard or turnips.
Anthracnose (fungus – Colletorichum higginsianum): Infected leaves are covered with small dry, circular, pale-gray to straw colored lesions (See Photo). Severe infection results in leaf death. On infected roots gray to tan lesions are formed. The bacterial soft rot organism often enters the lesions as a secondary infection and causes additional loss. The fungus overwinters in fallen leaves, on volunteer plants and some weeds. It can be seed transmitted. Optimum fungus growth occurs in a temperature range from 79oF and 86oF. The presence of moisture is important in disease development. Long crop rotations, weed control, and a well drained soil are important to reducing losses to this fungus. [Table 5] Approved fungicides can also be used when the disease appears. [Table 2] Begin fungicide applications when symptoms first appear and continue at 7 to 10 day intervals as long as weather conditions favor disease development.
Alternaria Leaf Spot (fungus – Alternaria brassicae): Small circular spots with a yellow halo are formed on the upper leaf surface (See Photo). As the spot matures the center becomes sooty black with age. The black color is partially the result of spore buildup on the surface of the lesion. Lesions are characterized by concentric rings within the lesion. Roots may also become infected when leaves drop on the soil. The fungus grows best at warm to hot temperatures and in the presence of light rainfall and dew. Spores are carried under the seed coat. Cultural practices [Table 5] will help reduce losses to this fungus disease. Apply fungicides at first sign of leaf symptoms. [Table 3]
Downy Mildew (fungus – Peronospora parasitica): The disease is most damaging in the fall and late spring, early summer months. The first symptoms are small faint yellow spots on the upper foliage (See Photo). As they mature, fungus fruiting structures are visible on the underside of the leaf. This growth is especially noticeable in the morning following a period of mild temperatures, high humidity and no wind. Petiole and stem lesions are indistinct, elongated and gray to black in color. In rainy weather, bacterial soft rot may invade plants weakened by downy mildew. Fungicides can be used as preventative sprays. [Table 1] It is important to monitor fields closely for signs of disease build up. Follow cultural practices to reduce inoculum build up and spread. [Table 5]
Black Rot (bacterium – Xanthomonas campestris pv. campestris): Black rot infects a large number of cruciferous plants including several commercially important vegetables. Infected mustard leaves are marked by a “v” shaped lesion on the leaf margin (See Photo). The bacteria enters through the water pores at the leaf margin and then move downward in the vascular strands of the leaf. In black rot infected leaves, the vascular system appears to have black to dark gray strands running through the tissue. Once infected leaves rapidly collapse. Infected leaves after harvest continue to break down in storage. The bacterium is carried in seed and can survive for a short time in the soil. Disease spread within a field is by rain, irrigation water or mechanical injury. Follow a least a 1 year. Longer rotations with non-cruciferous crops are suggested if the disease has been especially severe. Cultural practices can be used to reduce infection. [Table 5] Apply bactericides at first sign of leaf infection. [Table 4] Repeat at weekly intervals as long as weather conditions favor disease development. Damage will be first visible on lower foliage.
Bacterial Leaf Spot (bacterium – Xanthomonas campestris pv. raphani): The bacteria that causes bacterial leaf spot is seedborne. Foliage symptoms are pinpoint, watersoaked, olive green lesions (See Photo). As they enlarge the lesions become translucent spots with watersoaked margins. The bacteria not only are spread on infected seed but can possibly survive in the soil and on crop residue for a short period of time. The bacteria are spread by splashing rain and equipment. Cultural practices that reduce the time foliage is wet help to reduce infection. [Table 5] Rotations with vegetables other than members of the crucifer family will help to reduce presence of the bacteria in the soil. Fungicides used for black rot control will also help to control this organism. [Table 4]
Turnip Mosaic (virus): Leaves of infected plants are mottled and distorted. The entire plant is stunted and yields are reduced (See Photo). Aphids transmit the virus from diseased plants and weeds to healthy plants (See Photo). Field sanitation is important. At the end of harvest, diseased plants or those plants not harvested should be shredded and plowed under or tilled to reduce inoculum increase in an area. [Table 5] If diseased plants are left in place, aphid will continue to increase and as feeding pressure increases, winged forms develop and spread the virus to nearby mustard or turnip plantings. Control cruciferous weeds in and around the field or garden. Volunteer mustard and turnip plants should be destroyed.
|Table 3. Fungicides Approved For The Control Of Alternaria Leaf Spot|
|Kocide DF||1 – 2 lb||7 – 10 day||Begin applications when symptoms are first observed.|
|Table 4. Bactericides Approved For The Control Of Black Rot Of Turnip and Mustard|
|Kocide DF||1 – 2 lb||7 – 10 day||Begin when symptoms are first observed.|