Mary Jo Frazier

In an effort to test a broad range of compounds for activity against silver scurf all of the same treatments were not included in each test. Several classes of fungicides and some experimental formulations were tested. Four treatments, Tops 2.5D, Tops-MZ, TBZ -MZ, and Captan-MZ, all registered for use in Idaho, were included in all three years' tests. Maxim was tested in 1993 and 1994 as a flowable formulation and in 1994 and 1995 as a dust formulation.

In 1993, when seed pieces were dug six weeks after planting and evaluated for disease, Tops-MZ, Imazalil plus Tops-MZ, Captan-MZ and Maxim provided good control of H. solani in the inoculated seed lot (Table 1). All treatments which included Imazalil had a significantly lower disease rating than the untreated control. The healthy control showed a low level of infection from contamination or a low level of infection that had no visible symptoms on the seed. With the naturally infected seed lot Tops-MZ, Imazalil plus Tops-MZ, and Maxim provided the highest level of control but Captan-MZ and all the Imazalil treatments also significantly reduced infection (Table 2). Disease ratings of the naturally infected seed were slightly higher than with the inoculated seed. The natural infection may have allowed the fungus to become established in the periderm of the tuber where it was more resistant to the fungicide treatment. The disease ratings on the seed pieces eleven weeks after planting were slightly lower than at the six week evaluation. Treatments which were seen to significantly reduced the disease in the six week evaluation also significantly reduced the disease rating in the eleven week evaluation. At eleven weeks after planting some transmission of the silver scurf fungus to the progeny tubers was seen. The fungicide treatments which showed good control on the seed piece showed significantly less disease in the progeny tubers. The same trend was evident at the harvest evaluation but there was an increase in the incidence and severity of the disease. This is consistent with other researchers' investigations which show increasing disease severity the longer the tubers remain in the field. The post storage evaluation showed very little increase in disease after storage. This is contrary to many investigators results which generally show an increase in silver scurf after storage. Factors which may have contributed to this lack of disease increase in storage are relatively cool storage temperature, adequate umidity control which eliminated free moisture on the tubers, and the filtering of the air supply to prevent cross contamination of samples.

Table 1. Disease rating for inoculated seed lot in 1993

Table 2. Disease rating for naturally infected seed in 1993

The disease rating on the seed was much lower in 1994 than in 1993. Transmission of the disease to progeny tubers was also reduced. This decrease in disease may be due to the method of seed inoculation. In 1994 seed was inoculated in December, 1993, and then stored at seed storage temperatures. This method of inoculation may not have allowed infection to readily occur. If the seed had been inoculated immediately after harvest and then held at relatively warm curing temperatures (12-15C) a more successful development of disease may have occurred. Weather variability between the two years may also have been a factor. In 1993 total precipitation was 1.4 cm the first 30 days following planting as compared to 3.7 cm in 1994. The mean soil temperature in 1993 was 14C for the first month contrasted to 12.7C in 1994. Fungicide treatments which provided a reduced disease rating and incidence in 1993 also reduced disease in 1994.

Table 3. Disease rating in 1994

Table 4. Disease rating and yield in 1995

Conclusions

At full stand establishment, no significant differences in plant populations were seen for any treatment. Plant emergence when compared over the three years of the study showed no significant effects due to seed treatment. Tops-MZ, Captan-MZ and Maxim seed treatments provided good control of H. solani on the seed. The noninoculated control showed a low level of infection, probably due to a low level of natural infection in the seed or to soil-borne inoculum. Silver scurf was observed on the progeny tubers eleven weeks after planting. The fungicide treatments which showed good control on the seed piece resulted in significantly less disease in the progeny tubers. The same trend was evident for the harvest evaluation, but there was an increase in the incidence and severity of the disease. This is consistent with previous research that shows increasing disease severity the longer the tubers remain in the field (Wilcockson et al., 1985). The post-storage evaluations showed only a slight increase in silver scurf severity after 5 months of storage. Other researchers have reported a significant increase in silver scurf after storage (Adams and Hide, 1980). The incubation of samples prior to disease evaluation made detection of all infection sites possible regardless of size. Factors that may have limited both spread and increase in severity of disease while in storage are relatively cool storage temperature, adequate humidity control (which eliminated free moisture on the tubers), and the filtering of the air supply to prevent contamination of samples. Treatment differences were consistent in all three years, however yearly differences varied, perhaps due to environmental factors (Hide and Adams, 1980).
The use of effective seed treatments significantly reduced the incidence and severity of silver scurf in progeny tubers. Reducing the severity of the disease at harvest with the use of seed treatments may limit the amount of disease development in storage. The development of resistance to TPM and TBZ in isolates of H. solani has become a serious problem. Resistance may develop to other seed treatment chemicals with single site modes of action if used in a widespread and exclusive manner. This tendency toward fungicide resistance may be combated by combining single site action fungicides with multi-site activity partners such as TPM-MZ. Alternatively, rotating the use of fungicides among different chemical classes in successive generations will help slow the build up of fungicide resistance (Hide and Hall, 1993). Control of silver scurf was superior when fungicide combinations were used. TPM-MZ, Captan-MZ, and fludioxonil are effective for control of silver scurf and provide alternatives to benzimidazoles alone.