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DYNAMICS OF FUSARIUM CAERULEUM AFTER PLANTING INFECTED OR CONTAMINATED SEED POTATO TUBERS
DYNAMICS OF FUSARIUM CAERULEUM AFTER PLANTING INFECTED OR CONTAMINATED SEED POTATO TUBERS
SJ WALE and RC CLAYTON
SAC, Ferguson Building, Craibstone Estate, Bucksburn, Aberdeen, AB21 9YA, UK
Background and objectives

Dry rot of potatoes, caused by the fungus Fusarium caeruleum, has become an increasing problem in susceptible potato varieties in Scotland. Its occurrence is sporadic and the need for control measures at harvest, including the use of fungicides, is difficult to predict. However, the risk of dry rot is high in the progeny crop when seed is planted from stocks in which the disease has already become established. The objective of this study was to monitor the fungus population after planting infected or contaminated tubers.


Materials and methods

In each of two seasons, a stock of potatoes in which dry rot was established was obtained and tubers showing dry rot lesions carefully separated by hand from uninfected tubers. Replicated field trials were planted using uninfected tubers where 0, 1, 2, 5, 10 or 20% were replaced by tubers showing lesions. A further treatment included 10% tubers replaced but all tubers were treated with Mazin (maneb + zinc oxide, Universal Crop Protection). Soil samples were taken at defined points in each plot before planting, at emergence, at full canopy and at harvest and the population of F. caeruleum determined by dilution and plating onto a selective medium 1. The methodology was validated by testing spiked soil samples. Emergence was counted and progeny tubers were lifted. Yields were measured and tubers stored for 4-5 months after which the incidence of dry rot was determined.


Results and conclusions

Emergence was most rapid in the treatment where uninfected tubers were not replaced, but even in the treatment where 20% of tubers were replaced final emergence was over 90%. In the first season, 100% emergence occurred in the treatment with Mazin. With each treatment the population of F. caeruleum rose rapidly after planting and subsequently fell to low numbers. Although this pattern was similar for each treatment, there was a temporal difference in pattern between treatments. The peak of propagule numbers was earlier in those treatments with 10 and 20% tubers replaced than in treatments with 0, 1, 2 or 5% tubers replaced. In 1996 the trial was lifted late and at lifting propagule numbers were low for all treatments and dry rot in the progeny tubers was similar for all treatments. In the 1997 trial, when the harvest was six weeks earlier, the decline of propagule numbers was not completed in the 0% replaced treatment at the date of lifting and the extent of dry rot in the progeny was higher than for other treatments.


After planting, infected tubers rot down completely releasing large numbers of spores into the soil. This causes a rise in propagule numbers from planting onwards. However, the viability of propagules appears to be relatively limited and numbers fall. Where tubers without infection (and assumed to be contaminated) are planted, mother tuber breakdown is much slower but it seems that the fungus is a major factor in the breakdown. The later breakdown and consequently later release of propagules of contaminated tubers was confirmed. In practice, these results demonstrate that for stocks where dry rot is established, a delayed harvest, allowing maximum spore decline and maximum skin set should result in a lower risk of dry rot in the progeny crop.


Reference 1. Jeffries CJ et al. (1984). Ann. appl. Biol. 105, 475-483.