America. The leaves become brown on the margin and die. It is caused by drought, and is more prevalent on light soils. Irrigation and selection of vigorous varieties, more care in cultivation, and fertilizing are suggested. At Wisconsin Experiment Station,' Green Mountain, Rural New Yorker No. 2, Everett's Heavy Weight, and Colossal proved most resistant in 1896.

Sun Scald.'-Its effect is similar to that of tip burn. It is more prevalent in the Southeastern United States, and is often noticed when long-continued damp weather is followed by several hot, bright days.

2. Weeds. These injure the plant by using water and other plant-food, crowding the plant, preventing the free circulation of air, and in these ways reducing the vitality and rendering the potato more subject to disease.

3

3. Diseases Due to Parasitic Fungi and Bacteria.—LATE BLIGHT OR ROT3 (Phytophthora infestans). -There is reason to believe that this disease has existed for ages in the western parts of South America, and was disseminated over Europe a long time before its presence was recognized. It seriously injured the crops of potatoes in the United States and Canada in 1843, and reappeared the following year. In July, 1845, it was first detected in Europe, in Belgium, and within two months thereafter it was recorded in England, Ireland, Scotland, France, Germany, Denmark, and Russia. Since that time it has never been entirely

1 Wis. Report, 1896, p. 240.

2 U. S. D. A. Farmers' Bul. 91, p. 10.

3 (N. Y.) Cornell Bul. 113, pp. 297-302. Vt. Bul. 49, pp. 90, 91; Bul. 72, p. 13. U. S. D. A. Farmers' Bul. 91, p. 8.

absent from the potato crops, although in some years it is not so destructive as in others.

The disease appears during damp, muggy weather in August and September. It is often noticed as small brownish spots on the lower leaves, which rapidly enlarge. In moist weather the edges of these spots, on the under surface of the leaf, appear to be covered with a white downy mildew. In dry weather this may be difficult to detect. Later the leaves appear as though burnt, and finally the whole plant, and in some cases the whole field, will become a putrid, offensive mass of decaying stems and leaves. The tubers may be attacked also, and rot in the field or in storage. Sometimes the disease runs a very rapid course, and a field will wilt down in twenty-four to forty-eight hours.

Cause. The cause is a parasitic fungus which completes its life history in four or five days or less. The whitish mold is made up of stalks bearing branches (Fig. 34). These bear spore cases (Figs. 34 and 35), which break up to form spores (Fig. 35). These spores send out small tubes (Figs. 35, 36, 37), which enter the potato leaf through a stomata, or breathing pore (Fig. 37), or penetrate the cell wall (Figs. 36, 37). The tubes spread in the walls of the leaf cells (Fig. 34) like mushroom spawn in a mushroom bed, utilizing the plant-food which should go to form tubers. At intervals they send out spore-bearing branches through the stomata (Fig. 34), which perpetuate the trouble. Unless the tubers are well covered with soil, the spores may fall on the ground, and, reaching the tubers, transmit the disease to them.

FIG. 34-SECTION OF A POTATO LEAF
(After Marshall Ward)

Showing the parts and the threads, or mycelium, of the blight or rot (Phy-
tophthora infestans) a-Epidermis, or outer cells. 6-Palisade cells, which
aid in giving rigidity and firmness to the leaf, and in the manufacture of
starch and other ingredients. c-Spongy tissue, showing cells and large
air
spaces between. d-The stomata, or breathing pores of the leaf, with
aerial branches of the parasite growing outward through them. e-The
spore sacs, or conidia, in which the spores, or seeds, are formed. ƒ-A pe-
culiar hair on the under surface of the leaf. The dots in the cells are the
chlorophyll granules, which give the green color to the leaf, and aid in the
production of starch. The dark parts of the tissue show where cells are
dying from the effects of the disease. Loss of cells means a reduction in
the amount of food prepared, and, consequently, reduced yield. In New
York alone the farmers lose $8,000,000 to $10,000,000 annually from diseases,
and because they do not spray. This is the most important disease preva-
lent at present.

Aids to Attack.-1. Flea-beetles puncture the leaves and furnish easy access for the spores to the inner parts of the leaf.

2. Humid, still days, with a temperature of about 73°

F. Above 78° F. and below 50° F. there is practically no germination of the spores.

Prevention.-1. Spraying with copper compounds— as, Bordeaux mixture, copper sulphate and soda mix

f g h i

j

k

d

FIG. 35-THE MATURA FION OF A SPORE SAC (CONIDIA) AND GERMINATION OF A SPORE (ZOOSPORE) OF ROT (Phytophthora infestans)

(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). e-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. such occasions the disease spreads rapidly.

On

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 mix

6. Having good drainage-both water and air drain