Diseases of the Potato

DISEASES OF THE POTATO Potato Blight or Late Blight.—This is the most serious of the many diseases to which the potato is heir. It is found in all the potato growing countries and causes more damage to the potato than all the other diseases combined ; it is caused by a parasitic fungus, Phytophthora infestans. It seems to have been first noticed in Europe and North America about the year 1840 and by 1845 it had become widespread. In 1846 the disease was so rampant in Ireland as to cause the potato crop to fail almost completely, with a resultant famine. The disease is markedly seasonal in its effect, but always present in Britain; wet seasons are particularly favourable to the disease, and half the crop may be lost if precautions are not taken, while in dry summers it may do little damage. The disease generally shows itself first as purplish brown or blackish patches on the leaves, often appearing first near the top or margin. In damp weather the under sides of the patches may show delicate white threads, the hyphae of the fungus ; under such conditions also the dark coloured patches spread rapidly and the whole foliage may be transformed into a moist black mass. Leaves badly attacked give off an offensive odour detectable at a considerable distance. The minute white threads which make up the body of the fungus branch and pro duce large numbers of pear-shaped structures, the spores of the fungus. These spores become detached and are carried by move ment of the air to other plants, where they germinate and the threads resulting pass through the stomatal pores of the leaf and each starts a new discoloured patch. Under suitable con ditions a new crop of spores may be produced a few hours after infection, so the fungus can spread with great rapidity. In addi tion to moisture, temperature is important.

Muggy conditions, i.e., warm, moist air with a mean temperature of about 70° F are very conducive to the spread of the disease, while a temperature above 77° F persisting for a few days will retard its development. Under wet conditions the haulms (stems) are attacked and the disease soon attacks the tubers, the spores being washed down into the soil. The tubers first show the affection in the form of discoloured rusty patches. If the attack is serious, other minute organisms invade the blighted tubers and it decays as an evil-smelling, slimy mass. There has been con siderable discussion as to the cause of the appearance of the disease year after year. It is now recognized that the trouble is due to the planting of diseased tubers. If the seed tubers are boxed the diseased tubers are likely to be recognized by their abnormally early or weak sprouting. Infection may also result

from diseased tubers left in the ground after digging.

Preventive Measures Against Blight.

There are no varieties which are completely immune to blight, though some show considerable resistance, e.g., in England those of the Presi dent type and Irish Chieftain. The only satisfactory commercial method of prevention is the use of a protective spray or dust. Copper sprays in the form of Bordeaux or Burgundy mixtures are commonly used. The former is made from copper sulphate, quicklime and water, while in the second the lime is replaced by washing soda. The aim of the application is to cover the surface of the leaves with a layer which will cause the death of spores there germinating. Two or three applications are usually given, the times depending on the locality. Spraying of ter an attack has occurred is of value, as it checks the spread of the disease and reduces the damage. Dusting with a dry powder containing copper salts has been tried but is not as effective as spraying.

Wart Disease.

This disease is also due to a fungus, Synchy trium endobioticum. It was discovered in 1896 in Upper Hungary, being supposed to be a new form of scab. The earliest definite record of it in England was in 1898, but it is quite possible that it had existed there for some time before. The disease generally shows itself in the tuber, though occasionally the stems and even the leaves show warts. The affected tubers bear warty outgrowths or become warty masses. The fungus causing the disease is simpler than that causing blight, for it possesses no threads. The warty tissue of the tuber shows thick walled sporangia which, reaching the soil, liberate minute free-swimming cells, zoospores; these penetrate some part of a young potato plant, usually one eye of the tuber, and develop there, causing the abnormal warty tissue. In the affected tissue thinner walled sporangia, the summer sporangia, are produced. These discharge to the exterior and liberate fresh zoospores, which cause new infections, and so the process is repeated. The zoospores may unite in pairs and then after infection produce the thick-walled resting sporangia and these, when they escape into the soil, are the source of infection for new crops. The disease is easily spread by infected soil and tubers, and the winter sporangia pass through the alimentary canal of animals unharmed. Contamination of the soil is very persistent and even if no potato crop is grown on such soil it has been known to remain infected for twelve years; in fact, the time required for a soil to lose infectious power is unknown.