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F. Above 78° F. and below 50° F. there is practically no germination of the spores.
Prevention.-1. Spraying with copper compoundsas, Bordeaux mixture, copper sulphate and soda mix
FIG. 35-THE MATURATION OF A SPORE SAC (CONIDIA) AND GERMINATION OF A SPORE (ZOOSPORE) OF ROT
(Phytophthora in festans)
(After Marshall Ward) a-Ripe spore sac in water, b—The protoplasmic contents break up into blocks and escape as kidney-shaped zoospores (c and d). -Each have two thread-like arms, called cilia, which are lost as the spore comes to rest (fand g); h, i, j, and k show stages of germination. Moist, warm, or still, muggy days are best for the growth and development of these spores. On
such occasions the disease spreads rapidly.
ture. If the surfaces of the leaves and stems be covered with a thin film of some copper compound, we either prevent the entrance or injure the vitality of the germinating spore tubes, so that the potatoes enjoy a certain degree of immunity from the disease.
This is the philosophy of the use of Bordeaux mixture. The degree of immunity varies with our ability to keep the whole of the plant covered with an armor of Bordeaux mixture. Plants half sprayed are not secure, as the disease can spread rapidly inside the plant. The plant must be completely coated all the time to be immune. This may be impossible when a plant is growing, but this is not the fault of the Bordeaux mixture. The more thoroughly and more frequently the spraying is done the better the chances of bringing the crop through. It will be seen that Bordeaux is but a preventive; it is not a cure. Hence, the poor results from spraying
FIG. 36— LONGITUDINAL after the disease has obtained a
SECTION OF A POTATO foothold. 2. Obtaining disease-resisting
MINATING SPORE OF ROT
(Phytophthora in festans) varieties, or changing the seed
(After Marshall Ward) if it has lost its resisting power.
3. Planting on fresh ground, and planting early.
case of a leaf, but here the 4. Giving good cultivation, germ tube has pierced the
cell wall, and is growing in and having a good rotation. the cell. In spraying, the
5. Destroying all refuse of ste potatoes.
6. Having good drainage—both water and air drain
STALK, SHOWING A GER
The number of stomata per square inch on a potato stem is much smaller than in the
stems should be coated with
age. Near woodland, where the air drainage is poor, the disease sprearls rapidly on damp or misty days. Land choked with weeds keeps the lower leaves and stalks damp, and more subject to attack.
FIG. 36–THE GERMINATING TUBE OF A SPORE OF ROT
(After Marshall Ward) This may enter a plant through a stomata, or breathing pore, as at a, or it may penetrate the cell wall, as at b. The maintenance of a coat of Bordeaux mixture all over the plant would check the growth of these spores.
7. Not digging until ten days after the vines die.
8. Getting potatoes out of the field as soon as dug, and never covering piles of potatoes with spore-laden haulm.
EARLY BLIGHT,' OR LEAF SPOT DISEASE (Macrosporium solani).—It is a fungus disease which appears usually in June to July, or ahead of the late blight. It does not generally attack vigorous plants. It spreads in warmer, drier weather than the late blight. It forms circular brown spots with target-like markings on the leaves. It enters the leaf through tissues weakened by other agents, as flea-beetles, etc. It does not attack the tubers directly, and never causes them to rot.
Preventives.—1. Spraying with Bordeaux mixture. 2. Selection of vigorous varieties. 3. Better tillage and fertilization.
POTATO ROSETTE (Rhizoctonia solani.')—This disease has been known since 1842, but it is only recently that it has caused considerable trouble. It is now well established all over the country, and in some places 90 per cent. of the tubers appear to be affected by it. It tends to cause the formation of an abnormal number of small tubers of no value. The stems show discolored decaying areas above ground and brown dead areas below, and the leaves tend to grow in rosette-like clusters. The resting spores live for several years in the soil, and the methods of infection are by seed potatoes, beet and mangold roots, dead potato stems, and some weeds; hence, fields should be kept clean. The disease attacks beets, mangolds, and clover. Soaking the seed in formalin will destroy the spores on the potatoes, but is of no value if the soil is infected. Planting sound
1 Vt. Bul. 49, pp. 91-96; Bul. 72, pp 16–25. U. S. D. A. Farmers' Bul. 15, pp. 4, 5; Bul. 91, p. 5. (N. Y.) Geneva Bul. 123, p. 236. N. H. Bul. 45, pp. 50, 56. Tex. Bul. 42, p. 923.
2 (N. Y.) Cornell and Geneva Bul. 186. Col. Bul. 69, 70. Ohio Bul. 139, 145.
tubers and a good rotation of crops will aid in combatting the trouble.
SCAB (Oopsora scabies Than.).—Thaxter has shown that this fungus is the chief cause of scab (Fig. 38), although Rozé claims that the primary cause is bacterial, and that the fungus Oospora scabies and other organisms follow, causing the familiar rough and cankerous appearance of scab. Other causes are also given. An enormous amount of work has been expended on this disease, and still no absolute preventive is known if the land is inoculated with the trouble.
Treatment. Of a large number of substances used for treating the seed potatoes, soaking them in a solution of formalin, I pound to 30 gallons of water, for 2 hours is the most effective. Soak the potatoes before cutting them, and if they are not planted at once spread them thinly to dry. If left in bags they will heat and the buds be ruined. After soaking two or three lots of potatoes the solution should be changed, as it loses its efficacy. A big cheese-vat or sheep-dipping vat, in which several bags may be placed at a time, is useful. A small block and tackle will enable one man to lift large bags in and out of the vat, and suspend them to permit of some drainage.
The following points are of importance.
An acid condition of the soil is injurious to the growth of scab. Lime, wood ashes, and barn manure aid the growth of scab, while sulphate of ammonia, muriate of potash, sulphate of potash, kainit, acid phosphate, and dissolved bone render the soil less favorable
i W. Va. Special Bul. 44, pp. 285-6.