BEFORE USING ANY PESTICIDE STOP READ THE LABEL I Prepared by Furney A. Todd, Extension Professor of Plant Pathology North Carolina State University at

Transcription

1 CONTROL BLACK IN BU RLEY / TOBACCO

2 BEFORE USING ANY PESTICIDE STOP READ THE LABEL I Prepared by Furney A. Todd, Extension Professor of Plant Pathology North Carolina State University at Raleigh and the N. C. Department of Agriculture, Cooperating. State College Station. Raleigh N. C., George Hyatt, Jr., Director. Distributed in furtherance of the Acts of Congress of May 8 and June 30, March, 1966 Ext. Leaflet No. 116

3 CONTROL BLACK ROOT ROT IN BURLEY TOBACCO Black root rot is one of the oldest known diseases of tobacco and is found in all major tobacco-growing regions of the world. Previous to the development of resistant burley tobacco varieties, black root rot caused severe losses to tobacco growers in North Carolina. Even with the use of resistant varieties, some stunting of the crop has been observed for many years; however the loss has varied considerably from Season to season. Severe damage occurred in 1958, 1961, 1964 and 1965, especially in Buncombe, Haywood, Madison and Yancey Counties. Black root rot will continue to cause serious damage to burley tobacco even with the use of the moderately resistant varieties unless growers practice additional methods of control such as crop rotation, proper application of manure and the proper handling of winter cover crops. CAUSE Black root rot is caused by a fungus named Thielaviopsz's basicola. This fungus is present in most fields where burley tobacco is grown in this state and can live in the soil for an indefinite period of time even in the absence of tobacco. Tobacco is not the only crop susceptible to black root rot. Reports show that this fungus will attack 120 different species of plants, including most legumes, such as garden peas, soybeans, field peas, clovers and alfalfa. Cereal crops are usually regarded as resistant. HOW TO RECOGNIZE BLACK ROOT ROT In the plant bed. Black root rot is not easy to recognize in the plant bed; however, the occurrence of stunted, light green to yellow plants indicates the presence of this disease. An examination of the root system will help in recognizing the disease. Normally, young roots of tobacco plants are white, but on blackroot-rot-diseased plants many roots are brown to black, either in part or throughout the total length of the root system. The smaller roots are usually rotted throughout but on large roots the diseased areas may occur as brown to black irregularly shaped spots.

4 In the field. Stunted plant growth is the most characteristic symptom of this disease in the field (Fig. 1). Stunting is usually more prominent during the early part of the growing season. The extent of stunting varies considerably from field to field. Some fields may be affected throughout, while others only in spots. Diseased plants in affected areas are often uneven in development. For example, some plants may be only knee-high at flowering, with others in the same area almost normal in height. On hot, dry days the leaves on diseased plants Wilt more quickly than do those on healthy plants. The above-ground symptoms of black-root rot-infected plants can be confused with those of other diseases. Examination of the root systems of suspected plants is necessary to determine positively if black root rot is present. The most conspicuous symptom on the roots of infected plants is the brown to black discoloration (Fig. 2). Many feeder roots are destroyed, resulting in a much reduced root system. The small roots are usually black and may pull off when the plant is removed from the soil. Brown to black spots are usually found on the surface of larger roots. Microscopic examination of diseased root tissue for the presence of the black-root-rot fungus is often necessary for positive diagnosis. Below Ground Symptom. Black Root Rot Reduces Root Growth. Left to Right very severe, severe, moderate, slight, and very slight damage as compared to healthy root system (right).

5 Above Ground Symptom. Stunted growth, especially during early part of growing season, is characteristic of the black root rot disease. Crop performance is sometimes reduced by $400 to $500 per acre.

6 EFFECT OF ENVIRONMENTAL FACTORS ON DEVELOPMENT OF BLACK ROOT ROT Temperature. Temperature is probably the most important environmental factor affecting the severity of black root rot 1n tobacco. Damage is usually most serious during seasons characterized by late springs. The extent of damage caused by the disease can usually be correlated with May and June temperatures. If the mean temperature during May is 4-6 F below normal, the damage is usually serious. On the other hand, if mean May temperatures are 4-6 F above normal, little stunting usually develops in the presently used varieties. Research workers have reported that this fungus injures tobacco if soil temperatures are below 70 F. When the soil temperature is above 78 F, there is seldom any damage. Moisture. Black root rot appears to cause more damage during seasons characterized by early, heavy precipitation. Also, most damage usually occurs on heavy, poorly drained soils. Such soils should be avoided for plant beds and field plantings. CONTROL Best control of black root rot can be obtained by planning a complete program, which involves resistant varieties, proper plant bed management, crop rotation proper use of winter cover crops and manure, control of insects, and proper application of fertilizer. No single practice will give adequate control under favorable conditions. In plant beds. The first step in the control of black root rot to is produce disease-free transplants. This can best be plished accom- by selecting a medium to light, well-drained soil free the of black-root-rot fungus. If the fungus is present in treat the soil, with methyl bromide at the rate of 1 lb. per 100 sq. Calcium ft. cyanamide will not control this disease. In the field. Resistant varieties. All presently used varieties resistance carry to black some root rot. Under some conditions ance this appears resist- adequate; however, under other cially conditions, where espe- no rotation is used and during soil seasons temperature where the is below normal, the level of adequate resistance to take is not care of the problem in many fields. published Very little information is available on the level of resistance black-root-rot carried in the commonly grown varieties tobacco. of burley However, limited data indicate that more some resistant varieties than are others. See following list: LEVEL OF RESISTANCE TO BLACK Low ROOT ROT Low-Moderate Burley lla Moderate *MS Burley 21 x Ky. High 10 Kentucky Burley Burley *MS 49 Burley 21 x L8 *MS Burley Burley 21 x 21 Ky. 12 Kentucky 12 *MS Ky. 12 Burley x L8 37 Kentucky 16 *M8 L8 x Burlevy 37 * Indicates F1 Hybrid 6

7 CROP ROTATION The use of crop rotation is a most important part of a blackroot-rot control program. Many investigators believe that if burley tobacco growers would practice rotation the loss to this disease could be greatly reduced. The results from demonstrations conducted in the burley area in recent years confirm this belief. Care should be exercised in selecting crops for rotations to reduce black root rot. Commonly grown crops that are resistant to black root rot and can safely be used in rotation with burley tobacco are fescue, oats, wheat, barley, orchard grass, blue grass and milo. Crops that should not be included because they are susceptible to the black root rot fungus and would tend to build up the disease rather than reduce it include alfalfa, all clovers, soybeans, peas, garden beans and most other legumes. Length of rotation. Best results are obtained with a long rotation 2 to 3 years between crops of tobacco; however, a two-year rotation where only one alternate crop is allowed between crops of tobacco is far superior to no rotation. If land is available, plan a two- to three-year rotation. If land is scarce, work out a twoyear plan. Here are a few sample rotations that should be effective in reducing the incidence of black root rot in burley tobacco: TWO-Y EAR ROTATION First Year Second Year Corn Tobacco Small groin + weeds Tobacco TH REE-YEAR ROTATION First Year Second Year Third Year Small grain*, fescue Fescue Tobacco Small grain*, orchard grass Orchard grass Tobacco Orchard grass Orchard grass Tobacco Fescue Fescue Tobacco Corn Corn Tobacco FOUR-YEAR ROTATION First Year Second Year Third Year Fourth Year Small grain*, Fescue Fescue Tobacco fescue Small grain*, Orchard grass Orchard gross Tobacco orchard grass Corn Small grain, Orchard grass Tobacco orchard grass Corn Corn Corn Tobacco *Small grain optional Note: Certain herbicides, particularly Atrazine & Simazine, should not be used on corn that will be followed.by tobacco. 7

8 CHEMICAL CONTROL The results of recent tests indicate that the use of chemical soil treatment may be effective in reducing the incidence of black root rot in burley tobacco. Several soil treatments appear promising including Telone, Telone PEG and Vorlex when used in combination with resistant varieties. Chemical soil treatments tested thus far do not reduce the incidence of the disease low enough to permit the planting of susceptible varieties such as Judy Pride. Telone has been used for many years for nematode control in tobacco and has been included in black root rot tests for the past two years. The use of this fumigant has increased acre value $200 to $300 above the untreated check in black root rot problem fields. Telone is used at the rate of 8 gallons per acre row treatment or 16 gallons as a broadcast treatment and should be applied six to eight inches below the soil level for best results. A two to three weeks waiting period should be allowed between time of applying treatment and transplanting crop to avoid fumigant injury. Additional information is needed on performance of Telone PEG and Vorlex before suggesting their use on burley tobacco. OTHER FACTORS THAT MIGHT HELP IN REDUCING LOSS TO BLACK ROOT ROT 1. High soil ph favors the development of black root rot. For best control, keep the soil ph below 5.6. It would be advisable to check the soil ph and exercise caution in the use of lime on tobacco fields. 2. Avoid excessive use of manure or humus. The black-root rot fungus may be favored by high amounts of manure and humus in the soil. 3. Turn cover crops early. It would be advisable to plow under winter cover crops oats, barley and rye well in advance of transplanting. Research indicates that a toxin can be produced when these crops decay and this toxin apparently provides suitable conditions for root decay and black-root-rot development. 4. Exercise care in application of fertilizer. Use broadcast method of application or apply fertilizer deep in the row so that the plant roots are not injured. 5. Plow soil when right amount of moisture is present. Plowing the soil wet temporarily changes the structure of the soil to the extent that it often becomes compact. Stunted tobacco with black-root-rot symptoms has been observed on many fields where the soil was tight and compact, which probably resulted from plowing the land too wet.