Control of Silver Scurf on Potato by Dusting or Spraying Seed Tubers with Fungicides Before Planting

American Journal of Potato Research, Jul/Aug 2004 by (Lahkim), Leah Tsror, Peretz-Alon, Itzahk

Yield Assessment

Total yield was determined for each replication in a sample taken from 6 m of two rows (12 nWreplication). Tubers were harvested by hand, and after three weeks of cool storage (at 4 C with circulating air) graded to three classes (55 mm) and weighed.

Disease Evaluation

Silver scurf levels on seed tubers were evaluated in a sample of 200 tubers, whereas disease evaluation on daughter tubers was carried out on the same samples that were taken for yield assessment. The tubers were washed in running tap water and assessed visually for silver scurf according to four categories: nil, low (25%) (Tsror et al. 1999). An index of severity of silver scurf was calculated using the following formula: I = (N1 x 2 Nm x 5 Nh x 7)/N, where N1, Nm and Nh are the numbers of infected tubers at low, moderate and high levels, respectively, and N is the total number of tested tubers. The control efficiencies were calculated as follows: 100% - [(disease incidence (%) in each treatment/disease incidence (%) in the control) x 100].

Statistical Analysis

Disease incidence and tuber yield were evaluated first by two-way ANOVA to test whether there was a significant interaction between site and treatment. Since there were no interactions, means of three sites per each treatment were subjected to one-way analysis of variance (ANOVA), except for iprodione, where interaction was obtained and the combined treatments which were different at each site. Means were separated using Student's multiple range test. Data taken as percentages were transformed by arcsine before analysis. All analyses were performed with the StatSoft program (2300 E 14th St. Tulsa, OK) at P

RESULTS AND DISCUSSION

The present study demonstrated that silver scurf incidence and severity on daughter tubers was effectively reduced by a single pre-planting treatment of various fungicides applied to potato seed tubers by either LV spray or dusting. The incidence and the severity were significantly reduced by all treatments in the three experimental locations, with the exception of fluazinam, which was ineffective (Table 2). Among the fungicides tested in these experiments, the most effective were fludioxonil and prochloraz-Zn applied as a LV spray (control efficiency of 88% and 82%, respectively, as compared with the control), and propineb and mancidan applied as a dust treatment (control efficiency of 80% and 77%, respectively). Azoxystrobin and imazalil treatments were less effective with control efficiencies of 68% and 43%, respectively. Iprodine was less effective in one site (Urim) than in the two other sites (Table 2). None of the various treatments affected crop yields at any of the three experimental sites (data not shown).

Although daughter tubers were harvested earlier in Experiment III (Kibbutz Urim), resulting in relatively lower disease levels (incidence and severity index), similar control efficiency by the different fungicides was observed. The development of silver scurf in the three locations was different, and although the same lot of infected seed tubers was used, the disease severity index was highest (on average 1.7) at B'sor, with a lower control efficiency (on average 60%), compared with the other two locations, where the average efficiency of control was 70% in Saad and 71% in Urim. When the conditions for transmission and development of silver scurf become more favorable, the effectiveness of a seed treatment is likely to diminish, due either to enhanced development of the fungus on the seed tubers, or to the particle size of the soil allowing easier movement of the conidia to the daughter tuber. Soilborne inoculum may also have been a factor influencing disease levels at the various sites.