Spraying of Potatoes to Prevent Late Blight

New Zealand Journal of Agriculture, Volume 89, Issue 5, 15 November 1954, Page 487

Spraying of Potatoes to Prevent Late Blight

INFECTION of potato crops with the ■ fungous disease late blight (Phytophthora infestans) has been more prevalent in some districts in the past two seasons and has caused a considerable reduction in yield, particularly in the North Island. There is no cure for late blight, but preventive methods are described here by O. G. Moore, Fields Instructor, Department of Agriculture, Wellington.

THE disease attacks the plant in two through the foliage and stems and through the tubers. Leaf infection is indicated in the initial stages by dark patches which on the under surface of the leaf are surrounded by distinct whitish haloes. Similar dark patches appear on the stems when these become infected. Once a crop is attacked the disease spreads rapidly if conditions are humid and will soon destroy all .the leaves and finally the whole plant. If the spores of the fungus wash down through the soil and reach the tuber, tuber infection can follow. Diseased tubers show brown patches just beneath the skin. They are not suitable for storing owing to the rapid collapse of the diseased tissue and the likelihood of secondary infection with wet rots.

Preventive Treatment

There is no cure for late blight. The only treatment is to spray the crop to prevent. infection and, if infection is present, to combat its spread to healthy parts of the plants. It is not possible to say exactly when spraying of the crop should begin, but as a general recommendation it should be about 2 weeks before the disease normally appears. According to English literature late blight can be expected to develop when there have been a minimum temperature of 50 degrees F. and a relative humidity not falling below 75 per cent, for at least 2 days. When these two conditions occur the careful grower will arrange to spray within the following week. The number and frequency of sprayings will depend largely on the weather. The aim is to keep a coating of the spray material over the leaves, particularly the under surfaces, and so prevent entry of the spores. In most districts sprayings at 2- to 3-weekly intervals during the critical period of blight attack are sufficient to maintain this protection, but if rain is heavy between the planned times of application, it is a wise precaution to spray immediately after the rain.

Spray Materials

The spray materials used most widely in New Zealand for combat of late blight all contain copper compounds. They are Bordeaux mixture, Burgundy mixture, and proprietary formulations containing copper oxychloride. Bordeaux and Burgundy spray mixtures are among the oldest sprays known and are yet among the best. Copper sulphate and hydrated lime are used for Bordeaux mixture and copper sulphate and washing soda for Burgundy mixture. The choice of one

or the other of these two sprays is a matter of personal preference. Some growers think that Bordeaux mixture is less likely to injure the foliage and others claim that Burgundy mixture adheres better to the foliage and is generally easier to use, as it is not so liable to clog spray nozzles. Formulations for these two sprays vary widely and are subject again to individual preferences as well, as district practices. In New Zealand, Pukekohe is the potato growing area where spraying as a protection against late blight is most widely practised, and the quantities of each ingredient used most commonly in that district, based on information supplied by the local Instructor in Agriculture, are given below: — Bordeaux mixture: 41b. of copper, sulphate, 41b. of hydrated lime, and 40 gallons of water. Burgundy mixture: 41b. of copper sulphate, 41b. of washing soda, and 40 gallons of water. Some growers prefer to do their initial sprayings with mixtures with a lower proportion of copper sulphate .to lime or soda and then do further sprayings with concentrations as described. With this procedure they claim that damage to foliage is less likely. Examples of these solutions as sometimes used for initial sprayings are: 31b. of copper sulphate, 41b. of hydrated lime or washing soda, and 40 gallons of water; or 41b. of copper sulphate, 51b. of hydrated lime or washing soda, and 40 gallons of water. Mixing of Sprays The physical properties and powers of adhesion to the plant of Bordeaux mixture and Burgundy mixture are said by some scientific workers to be influenced by the methods employed in combining the copper sulphate with the lime or with the soda. Field trials conducted in New Zealand, however, indicate that comparable fungicidal results can be expected ' with all

methods of preparation except where a concentrated solution of copper sulphate is added to a concentrated suspension of lime or solution of soda and vice versa, or where the concentrated solutions of both ingredients are simultaneously added to the quantity of water required for diluon‘ Briefly, the various methods of preparation of both types of sprays may be summarised as follows, the particular method to be employed being left to 4 the user. L Add the concentrated suspension of lime (consistency of cream) or the concentrated solution of washing soda SmiU dilUted solution ° f coppei suipnaie. K^ d cnc£or> r solution dilutedl solution ol 01 llme or dllutect solution oi 3 ‘ Add to one another diluted sol “- tions of each ingredient. s imuSaneously into^a 6 thir^veS??* 8 s mult aneousiy into a third vessel, Irrespective of the method of preparation the spray should be stirred constantly while mixing is taking place and also thoroughly during use. T . . . „„„„„„ . 1S to prepare both these sprays m earthenware, glass, or wooden containers, and both should al Possible after mixi^’ otherwise damage to foliage is 11K eiy. The recommended proprietary preparations containing copper oxychloride are certified by the Plant Diseases Division of the Department o f Scientific and Industrial Research an a are verv eas ii v mixed at the Strengths indicated Y on Sir eontainers T , . ' . , , ls mn e «oii«r/° U ? d a PP roxi_ mately 100 gallons of spray per acre for afoquate coverage °f well-developed plants.