Scottish Agricultural College, Craibstone Estate, Bucksburn, Aberdeen AB21 9YA, UK

Background and objectives Silver scurf, caused by Helminthosporium solani has a significant effect on the visual quality of potatoes, particularly those for sale in pre-pack or seed markets. Pressures to reduce fungicide use in production have resulted in only two key methods for disease control in store being available, namely manipulation of temperature and free surface moisture. This study provides data on infection and disease development for incorporation into computer-driven store management systems.

Materials and methods
Experimental chambers in which condensation and temperature could be carefully controlled were constructed and were proven to closely simulate the conditions encountered in commercial potato stores [1]. The chambers were used to subject infected potatoes to durations of condensation (from 0 to 24 ;h) at 5, 10 and 1C. In addition, inoculated healthy tubers were removed and sectioned sequentially throughout experiments in which free surface moisture was present on tubers so that the conditions which trigger infection by H. ;solani could be established. An assessment scheme was devised based on the frequency of different morphological stages of spores (ungerminated, germinated, penetrating and ramifying) on the tuber surface and compared with visual assessment of symptoms after long-term storage.

Results and conclusions
Disease development, measured as the change in sporulation capacity and per cent surface area infected was related to duration of condensation, temperature and the interaction of the two. At 5C, 1 ;h of condensation caused a significant (P<0.001) increase of approximately 100% in sporulation capacity relative to untreated controls. At this lower temperature, disease development was not increased further by longer condensation events. At 1C longer condensation events resulted in greater increases in disease development such that 1-, 2- and 3-hour condensation events caused three-, five- and eight-fold increases in sporulation capacity respectively.

Observation of spores inoculated onto healthy tubers showed that more than 60% of spores germinated within 2 ;h in the presence of free moisture. There appeared to be no tropic control of germ tubes and only 14% made contact with the potato epidermis. Of those, only half penetrated the epidermis and produced new lesions. Penetration occurred within the first 6 ;h of free surface moisture and the frequency did not increase thereafter. The frequency of penetration correlated highly with the number of infection sites per tuber assessed visually after long term storage.

These studies have demonstrated the need to limit condensation and free surface moisture during potato storage, particularly after store loading where higher temperatures are required for wound healing. In concurrent studies, algorithms were developed to control condensation in potato stores and have been successfully incorporated into computerized store-management systems.

1. Hardy CE, Burgess PJ, Pringle RT, 1997. Potato Research 40, 169-180.