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Early blight is caused by a fungus disease that attacks the leaves and stems of the potato plant. In the early stages, small dark brown or black spots that are circular or irregular in shape appear first on the lower leaves. These spots have a ringed or target-like type of appearance. They enlarge and also become more numerous as the season progresses. The leaves die when the spots become numerous. Sunken black areas that are irregular in shape and may be as large as an inch in diameter develop on infected tubers. As a result, these tubers are usually not marketable for human consumption.
Early blight develops most rapidly under moist, humid conditions. It usually makes its appearance in potato fields about the time the tubers are forming.
How to control early blight of disease potato?
Early blight of disease potato does not spread so rapidly as late blight, but early blight is more difficult to control. Bordeaux is prepared by mixing 10 pounds of copper sulphate (blue vitriol), 7 pounds of hydrated lime, and 100 gallons of water.
Although Bordeaux mixture is still use in the control early blight of disease potato, better results are now obtained with carbamate compounds such as nabam zinc sulphate, zineb, maneb, or tank mixed ziram. These chemicals can be purchased under the following trade names:
Nabam…………………………… dithane D-14, parzate liquid, thiodow liquid, ortho nabam liquid.
Zineb………………………………dithane Z-78, parzate (powder), ortho zineb.
Maneb…………………………......dithane M-22, manzate,
Ziram (tank mixed)…………….......coro-SDD
Applications should be made controlling to manufacturer’s directions.
It is god practice to keep all new growth protected with the spray material and to renew the application on the older growth. Spraying usually begins when the potato plants are 4 to 6 inches tall with applications spaced at ten-day intervals for the rest of the growing period. During periods of wet water, it may be necessary to spray every five days to maintain the necessary control. Actually, the number of applications necessary to control early blight of disease potato varies with the prevailing climatic conditions and must be determined by the grower himself. During some years and in some localities, five or six applications will suffice; under other conditions 10 or 12 are necessary. In some years, spraying for early blight is not necessary at all.
To provide the necessary protection, an application of 60 to 75 gallons per acre of spray of the above formulations is required when the potato plants are small and 100 to 125 gallons when they are large.
Some growers prefer to use dusts rather than sprays to control early blight of disease potato. Spraying is less expensive, probably somewhat more effective, and is not as dependent on weather conditions for proper application. However, dusting requires less expensive equipment and can be done with greater ease than spraying. The dust is made by mixing 20 pounds of monohydrated copper sulphate dust with 80 pounds of hydrated lime. Applications of the copper-lime dust vary from 35 to 45 pounds per acre depending upon the size of the plants. Carbamate dusts also provide effective control early blight of disease potato. Usually the same number of applications of dust as spray are needed during the season. If the dust is applied when the leaves are dry, it is likely to be blown off. Thus, it is best to apply dusts early in the morning or late in the afternoon or early evening when the plants are wet with dew.
Crop rotation also aids in controlling early blight of disease potato, inasmuch as the fungus overwinters on the stems and leaves of the old potato plants.