POTATO BLIGHT

Evening Star, Issue 22631, 24 April 1937, Page 23

POTATO BLIGHT

METHODS OF TREATMENT So much damage is done every year bv the dreaded potato blight that I cannot do better than write a little about this disease, its life history and the various ways of reducing its onslaught on our potato crops (says the gardening contributor of the ‘ Southland Times’)- , . To begin with, we must delve a little into'plant pathology. The diseases of plants are divided into two groups(l) those caused by “ non-living ” environment, such as soil and temperature; (2) those caused by parasites (living organisms). The parasites may again be divided into two classes, animal parasites and plant parasites. The plant parasites or fungi all belong to the plant kingdom and come under the division Thalophyta. There are bebetween four and five thousand species of fungi, and each year research workers on plant diseases add more to our Jist. Fungi show a complete absence of chlorophyll, and cannot therefore make use of the carbon in the atmosphere. They must obtain it from some organic compound, such as plants or animals, or from the humus in the soil. > Briefly stated, a fungus consists of a series of very fine interwoven threads called hyphae and spores for reproduction. The mycelium consists of all the hyphae. The spores are very minute bodies borne singly on a hypha, or in a group, as in the toadstool. They vary in size, shape, and colour, and the classification of fungi is generally based on the way the spores are borne, their shape, and colour. The potato blight belongs to a group of fungi known as phycomycetes, all of which grow in an abundance of moisture. Phytophthora infestans is the name given to the potato blight itself, and we know that at the present time it is found in most countries throughout the world. In 1845 it was one of the chief causes of the Irish famine. It occurs in two distinct ways—(l) on the foliage, (2) on the tubers. _ When occurring on the foliage it is seen as blackish spots in the summer. As a rule it does not appear till January or February, when the weather is close and muggy, thereby aiding its growth. It may be recognised by the aid of a hand lens, the blotches on the leaves having a ring of white round them; this is the mycelium. This blotch or disc _ spreads rapidly, wet weather especially favouring its growth. I have known in some cases where whole fields have been blackened in a week. The tubers of such plants are always small. . When blight occurs on the tubers the infection often takes place by spores which have fallen from the leaf and washed into the soil by rain. Brown-red stains appear underneath the skin of the tuber, due to the spreading of the mycelium. At one time it was thought that the disease traversed the stem from the leaves to the tubers, but this is a mistake.

PLANTING OF DISEASED TUBERS. Often the disease arises from the planting of diseased tubers. In such a case three things are liable to happen. Firstly, tubers may die; secondly, diseased plants come up, due to the infection of the tubers in the region of the “eyes.” The mycelium ‘ develops from the tubers up into the young growing shoots, which in all probability will die; but often not before spores are ripened to infect healthy plants nearby. Thirdly, healthy plants may appear, due to non-infection of that part of the tuber in the vicinity of the eyes.

The fungus is small, has non-septate mycelium, and has sexual reproduction. The spores are born on hyphae which comes out of the stomata of the leaf. The hyphae is rather stiff, often branching in groups of three or four. This is easily seen through a strong hand lens. At the extreme end of the hyphae small pear-shaped bodies can be seen; these are called conidia.

Now let us see how the sexual or resting spores are produced. On the sides of the mycelium, egg cells (oogonium) are produced, while on other branches of the mycelium, antheridia are borne. The antheridium stretches out, or grows towards the oogonium; the cell wall between the two eventually breaks down, and the whole of the cell contents of the antheridium fuses with the oogonium, thus giving rise to the sexual or resting spores. These spores are occasionally met with inside the “ shell ” of the old decayed tubers-

The above is a brief outline of this dreaded disease, very interesting like many fungus diseases-in its life history. The chief point which now concerns us is how to deal with it when our potato crops are attacked. The great remedy is the application of a fungicide in the form of a dry or wet spray. Liquid spray is, generally speaking, the best to use, though a little more expen sive. If a dry spray is used it is better applied in the early morning when the dew is still on the leaves. Burgundy mixture and Bordeaux mixture are the two wet sprays which may be used. Spray when the sun is not too hot.

Never use infected tubers for seed, grow only certified seed, and it is a good plan to properly green and sprout all tubers required for planting. If the disease appears late in the season cut off and burn all foliage, and avoid lifting the crop while any diseased leaves and tubers are about.