RE FALLOON, HM NOTT and RA GENET

New Zealand Institute for Crop and Food Research Ltd, Private Bag 4704, Christchurch, New Zealand

Background and objectives
Powdery scab (Spongospora subterranea f. ;sp. subterranea) is economically important wherever potatoes (Solanum tuberosum) are grown [1]. Until now, this disease was thought to have little impact on plant growth, with recognized deleterious effects being indirect reductions in potato tuber quality. These problems include the entry of secondary pre- and post-harvest tuber pathogens through powdery scab lesions [1] and the reduced acceptability of infected tubers for use as seed in crop establishment, for sale as high quality vegetables or for processing [1, 2]. In a recent field trial with two potato cultivars we measured total tuber yield increases of up to 28% when severe powdery scab was prevented using soil-applied chemicals (fluazinam and mancozeb) [2]. A glasshouse experiment was therefore carried out to determine if the disease can affect plant productivity.

Materials and methods
Tubers from a recently harvested seedline of cv. Rua were sorted into two categories, those without powdery scab lesions (100 tubers) and those with the disease (151 tubers). Powdery scab severity was assessed using a standardized key [3]. Tubers were cool-stored (5 ;months, 4øC) and were then allowed to develop sprouts (15 ;days, 18-24øC). Numbers of sprouts on all tubers were counted before planting each tuber in a pot containing potting soil and placing the pots in a glasshouse (12-24øC). One-third of the resulting plants from healthy and diseased seed tubers were each harvested 6, 8 or 14 ;weeks after planting, when plants were at vegetative, flowering or senescent growth stages respectively, and plant parameters were measured.

Results and conclusions
Powdery scab severity on the diseased seed tubers was between 3 and 47% of surface area infected (mean=10%). Almost half of the infected tubers had no sprouts at the time of planting (mean number sprouts/tuber=1.8), while all of the healthy tubers had at least one sprout (mean=5.4/tuber). Sixty per cent of the infected tubers produced plants, while plants grew from almost all (99%) of the healthy tubers. Plants from infected tubers had fewer stems (mean=3.1) than those from healthy tubers (5.7), lower fresh weights when vegetative (137 versus 191 ;g) and at flowering (150 vs. 210 ;g), and lower dry weights at these two growth stages (8.3 vs. 11.0 ;g and 7.5 vs. 11.8 ;g respectively). At senescence, plants from infected seed tubers produced fewer tubers (mean=8.1/plant) than those from healthy seed tubers (10.3), and lower total weight of tubers (87 vs. 106 ;g). The decreased yield was due to fewer tubers/plant on plants from infected seed, as plants from both categories produced tubers of similar weight (diseased mean=10.6 ;g/tuber, healthy=10.3).

These results demonstrate that powdery scab on cool-stored seed potato tubers can severely affect establishment, growth and yield of resulting plants confirming some of our field trial results [2]. In the present study, reductions in plant parameters due to powdery scab were 66% for numbers of sprouts on seed tubers, 39% for plant emergence, 45% for numbers of stems on resulting plants, 28-29% for plant fresh weight, and 19% for total tuber yield. This further demonstrates that powdery scab probably has greater consequences than indirect effects on tuber quality. Our results further emphasize the need for good control of powdery scab using appropriate disease resistance, chemical and cultural methods in an integrated disease management strategy [4].

References
1. Karling JS, 1968. The Plasmodiophorales. New York: Hafner Publishing Company, pp. 180-191.
2. Falloon RE, Wallace AR, Braithwaite M, Genet RA, Nott HM, Fletcher JD, Braam WF, 1996. New Zealand Journal of Crop and Horticultural Science 24, 341-353.
3. Falloon RE, Viljanen-Rollinson SLH, Coles GD, Poff JD, 1995. New Zealand Journal of Crop and Horticultural Science 23, 31-37.
4. Genet R, Falloon R, Braithwaite M, Wallace A, Fletcher J, Nott H, Braam F, 1996. Abstracts of papers of the 13th Triennial Conference of the European Association for Potato Research, Veldhoven, The Netherlands, July 1996.