Skin spot (Oospora pustulans) infection of the potato crop

Abstract

The potato industry in Scotland is based on the annual production of healthy seed, the bulk of which is sent to English markets. Within the last few years higher costs of production, marketing and transport have increased the price of seed potatoes to such a level that English growers have become much more critical of the quality of Scots seed, and many have turned to Irish and foreign sources of supply. To maintain interest in this it is therefore essential that Scottish growers make every effort to prevent poor quality seed reaching the market.

Skin spot is one disease which has caused numerous complaints from potential customers, particularly after the severe outbreaks in the spring of 1961 and 1962, which caused considerable blanking and delayed emergence in potato crops. As previously stated, tubers become. infected with the fungus at or be fore lifting, but no obvious symptoms become apparent until about February, and even then they may not be readily visible on unwashed tubers. Thus many consignments of seed are dispatched in apparently good condition and free from the disease, but by planting time are found to be severely infected.

Such a situation has led to the demand for an effective method of preventing the development of skin spot on seed tubers, and the experiments reported in this thesis are designed to investigate the possibility of controlling the incidence of the disease by both cultural and chemical methods. Three approaches to the problem have been investigated: (a) the effect of varying the environment in which the tubers are stored (after artificial inoculation). Incidence of the disease was studied after storage at high and low humidity under three temperature conditions:

(1) Continuous low temperature

(2) Variable temperature as found in farm bulk storage

(3) Continuous warm conditions

(b) chemical control of the fungus using· an organo-mercury dip, and a comparison with the effect of boxing. Both treatments were carried out before lifting, at lifting, and 1, 3, 6, 9, and (in the second year) 12 weeks. after lifting, and the subsequent development of skin spot: compared with the degree of infection on untreated tubers.

(c) investigation in to the effect of varying conditions at planting on the emergence of infected seed tubers, and on the infection which developed on the subsequent crop:

(1) by using two planting dates (2) by disinfection of seed immediately prior to planting.