POTATO GROWING.

Hauraki Plains Gazette, Volume XXXVI, Issue 4896, 28 October 1925, Page 4

POTATO GROWING.

THE “IRISH” BLIGHT. There is no. disease of the potato that causes such wholesale destruction of the crops as Late, or “Irish” blight when a serious visitation occurs. Last season it made its unwelcome appearance in some parts, but, fortunately, it was not as widespread as in 1910-11, and the losses were comparatively small. In the year mentioned the blight took a terrible toll and cost growers in this Dominion thousands of pounds sterling. Since then it h.as year by year beep a diminishing quantity. But its capacity for damage must not be ignored. Blighting begins at the tips or. edges of the leaflets and makes dark, greasy-looking, or water-spaked spots, usually involving one-third to onehalf of the leaflet when first noticed. The under-surface of a leaflet so affected appears filmy, or cobwebby, with the fruiting threads of .the fungus. Under wet conditions all tender parts of the tops blight and quickly rot away with characteristic rank odour. A blighted field assumes the appearance of being sttiuck by frost. In the first stages of rotting the affected .tubers show slightly sunken lead-coloured or pinkish areas. Sometimes these are mere depressions here and there, making .the potato look “hob-nailed.” Under moist conditions, or when dug in wet fields, the tubers may show greyish-white tufte 'of’ the fungus. These are fruiting threads, such as have been mentioned as occurring upon the leaves. Rotting is most serious with tubers grown in heavy soils. The flesh beneath the skin is j brown, and when the skin is scraped away the tell-tale brown discolouration of ‘he besh indicates .the blighted condition of the tuber. As the blight advances in the tuber more and more of the flesh is involved, and the colour of the rotted flesh changes to black. If storage conditions are dry .the tuber wizens into a mummy—“idry rot.” If the tubers are kept wet, or even moist, they rot with an evil-smelling, slimy rot —“wet? rot.”

The. disease is caused by a para’sitic fungus. Phytophthora intostans, which restricts its attack to plants of the potato family. The fungus is probably carried to the field as a lurking infection in practically every lot of .tubers planted. No experiments have been able to demonstrate that the fungus lives in the winter from crop to cron in the soil. The majority of sprouts that .arise from infected seed tubers are weak and subject to ratting in the ground, thus giving a poor stand. However, under favourable weather conditions, a few infected ■sprouts get above-.the ground and communicate the disease to nearby plante. Given favourable wteather conditions to promote! the spread and growth of the spores of the fungus in the first half of the growing season the disease spreads from plant to plant and becames established throughout the field. With such an entrenchment of the < funguis in the field, even moderately wet weather during the latter half of the season is sufficient to bring about a blight epidemic. To the farmer the bright appears very sudden in ite action, but in reality the fungus has been establishing itself for more than a month.

Plotting of the tubers comes about from the spores washed from the tops through the soil or shaken upon the tubers at digging time. It does not come about from the fungus working down the stalk, as is generally supposed. An examination of the blighted areas an .the tuber will show this beyond question. Occasionally the fungus spores w'ash down along the stem and cause rotting, in close setting varieties, which begins at the stem end of the tuber. A spore of this parasitic fungus carried to the tuber sprouts, if given favourable conditions of moisture and temperature, bares through the tskin. Once the fugus gets into the flesh of the tuber the advance is rapid. The browning of the flesh is s an indication of the activity of the parasite. The rapidity of the progress of the rbt depends upon the temperature. Moist conditions favour spore production and the spread of the rot.

Probably no commercial seed, stock available to the farmer isjtruly blight free, and . no practical method Las been devised of ridding seed of blight infestation. Tubers showing even a slight evidence of rot should be sorted out, not th.at this sorting will eliminate blight, but because sprouts from

such potatoes are notorious in giving a poor stand of. weak hills. Sound tubers from a, stock that has rotted from late blight are safe to plant, but rotted tubers are dangerous. The main control, measure against late blight constats of a thorough application of properly made Bordeaux, mixture. Plants should be sprayed every 10 ’days or two weeks, commencing when the tops are from 3in to 6in high. Farmers who regularly raise heavy crops should spray every year when the first half of the growing season is cbld and wet. If protective sprayings, have been neglected a crop can still :be saved if the blight is recognised when first starting. When the tops are blackened, .and the foliage wilted, nothing can be .accomplished by spraying, which seeks -to preyent wholesale leaf infection and prevents rotting of hte tubers, since it does away with the source of tuber infection, namely, the diseased leaves. The great economic waste .that arises from late blight is unnecessary, and could be avoided if farmers adopted a. rational spraying practice. Many growers say that it does not pay to spray, that it injures .the crop, etc;; but when the blight catches them they have a different tale to tell'. It is obvious that it. is profitable to take all reasonable precautions against the disease in preference to losing the whole of the crop, which is likely to occur when the disease is virulent in its attack, as it may be in any season. Immunity for a number of years fits no guarantee of immunity for an indefinite period.