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

ABSTRACT

Silver scurf of potato caused by Helminthosporium solani Dur. & Mont, is a blemish disease that has become economically important in recent years and is considered primarily seedborne. The objective of the present study was to compare the effectiveness of various fungicides, alone or in combination, applied to seed tubers prior to planting, in controlling silver scurf on the daughter tubers. Disease incidence and severity were significantly reduced by all treatments in three experimental locations, with the exception of fluazinam, which was not effective. 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) and propineb and mancidan applied as a dust treatment (control efficiency of 78% and 77%, respectively). Azoxystrobin and imazalil treatments were less effective with control efficiencies of 68% and 43%, respectively. None of the various treatments affected crop yields in the three experimental sites.

Accepted for publication 10 March 2004.

ADDITIONAL KEY WORDS: potato, Solatium tuberosum, Heliminthosporium solani, fungicides, LV spray, dusting, quality

INTRODUCTION

Silver scurf of potato caused by Helmintkospomum solani Dur. & Mont, is a blemish disease that has become economically important in recent years. This can be attributed to increased awareness, the increasing prevalence of washed pre-packed potatoes in which blemishes are more readily apparent (Errampalli et al. 2001; secor and Gudmestad 1999), resistance to thiabendazole fungicides (Bains et al. 1996; Hall and Hide 1994; Hide et al. 1988), and storage conditions with higher humidity (Brook et al. 1995; Secor and Gudmestad 1999).

The disease is characterized by a dark metallic discoloration of the periderm in irregular patterns. The lesions on the tuber increase in size, coalesce, and may cover a major portion of the tuber surface. Primary infection occurs in the field and secondary lesions develop from conidia dispersed during storage. The lesions reduce tuber quality of all market classes of potatoes, resulting in reduced consumer acceptance. The disease does not cause yield losses at harvest, but does cause weight loss of stored potatoes due to increased water loss (Errampalli 2001; Read and Hide 1984; Tsror (Lahkim) and Peretz-Alon 2002).

Silver scurf is considered primarily a seedbome disease, and seed tuber treatment is the most effective means of control. In general, very few fungicides are effective against the silver scurf pathogen (Errampalli et al. 2001). Even organomercurials were found to be effective only on fungal mycelium on the tuber surface; mycelium present beneath tuber skin was unaffected and sporulation resumed in the soil after planting or during storage (Firman and Alien 1995; Hide et al. 1988). Thiabendazole (TBZ) was effective until resistance was identified in the late 1970s (Errampalli et al. 2001). Chemical control of H. solani with fungicides other than TBZ was recently reviewed by Errampalli et al. (2001); imazalil, prochloraz, fludioxonil, and fenpiclonil were reported as efficient agents. The objective of the present study was to compare the effectiveness of various fungicides, alone or in combination, applied to seed tubers prior to planting in controlling silver scurf on the daughter tubers.

MATERIALS AND METHODS

Field Experiments

Three field experiments were carried out at three locations in northwestern Negev (southern Israel) in 1998. Potato crops were grown for four years prior the setup of trials in all sites. Experiment I was carried out in sandy soil at the B'sor experiment station, Experiment II in loess soil at Kibbutz Saad, and Experiment III in loess-sandy soil at Kibbutz Urim. Each experiment was conducted in a completely randomized block design with four replications (each 12 m in length along two rows).

Plant Material

Tubers for the experiment were taken from a certified potato seed lot of the highly susceptible cultivar Desiree imported from The Netherlands. Eighty percent of the seed tubers were naturally infected with H. solani, with disease severity index of 3.32 (on a scale of 1-7), calculated according the formula described in the following section on disease evaluation.

Seed Tuber Treatments

Seed tubers were treated with various fungicides as listed in Table 1, by manual dusting or by a low volume (LV) technique. The latter treatment was applied in an aerosol spray chamber, as rotating tubers carried on a roller conveyer, were sprayed with fungicide droplets of 90 �m in size (700-900 droplets/cm^sup 2^), produced by a rotary atomizer (Micron mini ulva^sup R^) (Institute of Agricultural Engineering, Agricultural Research Organization, Bet Dagan, Israel) positioned 60 cm above the tubers (Tsror and Peretz-Alon 2002). The throughput of potatoes on the conveyor belt at full capacity was 8 ton/h and the application rate of the sprayer was 1.2 L/ton seed tubers. After treatment, the tubers were stored for 3 months at 4 C with circulating air. Non-treated seed tubers served as control. The application rates used for the fungicides were based on preliminary examinations of different concentrations for each fungicide. The application rate of mancozeb in the combined treatment was three times less than it was applied alone in order to avoid overdose concentrations.