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Cornell Plant Disease Clinic


Lessons Learned from a Stressful 2001 Potato Season


Thomas A. Zitter, Dept. of Plant Pathology, and
Donald A. Halseth, Dept of Horticulture,
Cornell University, Ithaca, NY 14853

Issued May 15, 2002


Every season presents new challenges to potato growers (seed, fresh market and processing) in New York, and the 2001 season was no exception. Fortunately, late blight was not an issue in 2001, and therefore growers should be able to begin the season with good quality seed and a limited impact of disease originating from cull piles and volunteers.

Annually we conduct fungicide seed piece and foliar fungicide trials at research plots in Freeville, NY. Although the soil type at Freeville may be quite different from that on your own farm, we usually deal with similar environmental factors (temperature, humidity, and drought) that can impact potato production practices throughout the state. The 2001 growing season turned out to be very stressful for potato production, and this resulted in more potato disease problems than we normally encounter. In addition to the common occurrence of Rhizoctonia black scurf, Helminthosporium silver scurf, and early blight, we also observed an extensive amount of Colletotrichum black dot damage (Colletotrichum coccodes) to the foliage, stems, and tubers. When the influence of all of the diseases were taken together, their occurrence contributed to early vine death, poor tuber appearance, and poor stored product. By trialing an assortment of fungicide seed piece treatments and foliar fungicide sprays in 2001 in various combinations, we observed that choice of seed piece treatments and appropriate alternation of foliar fungicides can significantly influence the control of early blight, the lack of early vine death due to black dot infection, better tuber appearance at time of grading, and decidedly better quality of tubers after extended storage. Not all of the fungicides tested are currently labeled for use in New York (not approved by NY-DEC), but they have been included to show the range of materials tested, and which ones are worthy of future consideration.

Seed Piece and In-furrow Treatments

In 2001 we studied 6 different materials as seed piece or in-furrow applications made alone or in combination to both ‘Eva’ and ‘Chieftain’ seed (8 different treatments were studied). These included an untreated control, Messenger (applied as a dip treatment), Quadris in-furrow (0.958 fl oz/1000 ft row) (used in three different treatments); Blocker in-furrow (10 pt/A), Maxim MZ (0.50 lb/100 lb seed), T-22 (2.0 oz/100 lb seed) and Tops MZ (0.75 lb/100 lb seed). Fusarium seed piece decay was not a problem at planting, and normal emergence was noted. We did not do a destructive harvest to look for the amount of Rhizoctonia stolon burn, but with the exception of Messenger, all of the treatments used are known to control Rhizoctonia. Although a difference was noted in the number of stems per plant, this feature had no influence on total or marketable yields or the mean tuber number per linear foot for any of the treatments. In terms of the best tuber appearance at grading (tubers showing the least amount of black scurf, silver scurf and black dot discoloration), the best treatment numerically was the combination of Maxim MZ used on the seed and Blocker applied in-furrow. We feel that the overall benefit of this treatment was derived from Maxim MZ, since the same level of control was not achieved when Blocker was used by itself. Treatments that were not significantly different from the Maxim MZ treatment were those that received a Quadris in-furrow application. These results support our two previous years of testing with these products, and offer the best control when all three diseases are present. If your seed quality is suspect, and Fusarium dry rot is an issue, then the inclusion of mancozeb either in a mixture or alone is an important consideration.

Foliar Treatments

We normally do not think of foliar fungicide applications as adding dramatically to overall vine health, tuber appearance, and storability, but our 2001 results have led to just that conclusion. By tracking the use of 8 different foliar fungicide schedules applied to the same seed piece/in-furrow treatments mentioned above, we have learned which combinations worked the best in 2001 for early blight and black dot control. Black dot, caused by the seed and soilborne fungus C. coccodes, can be recognized by abundant black dot-like sclerotia found on tubers, stolons, roots, and on stems both above and below ground. However, the symptoms on affected foliage can be confused with early blight and early dying attributed to Verticillium wilt. In fact, we noted foliar symptoms of both early blight and black dot by August 7 in our plots. Early blight was expected since we purposely inoculated the plots twice in July. Black dot infections occurred from natural inoculum (it is a common soilborne pathogen in most agricultural fields). By the end of August, the plots were under so much stress that vines and foliage in several treatments were nearly dead from black dot, so much so that we could no longer distinguish between damage caused by early blight vs. black dot. Black dot was confirmed by rating the occurrence of sclerotia on potato stems in all treatments. The strobilurin fungicides azoxystrobin (Quadris) and pyraclostrobin (Headline) are known to be effective for both early blight and black dot control, and when alternated with chlorothalonil (Bravo), provided the best control of both diseases in our plots. The use of these sprays also contributed to less black dot on the tuber surface, which was noted at the time of grading, and again when the tubers were assayed for black dot following 5 months of storage. What was particularly noteworthy was the extensive amount of sunken lesions observed on the tuber surface for most treatments (see Figures 1 and 2). These lesions, observed on both the stem end and spread over the main tuber surface, although not penetrating very deep into the tuber cortex, made the tubers completely unsaleable. Tubers with these symptoms also suffered from considerable shrinkage. We suspect that silver scurf, perhaps aided by the presence of black dot, was responsible for the lesions. The treatment that consistently ranked the best for both ‘Eva’ and ‘Chieftain’ had received the Maxim MZ seed treatment (with Blocker in-furrow) and the alternation of Headline with Bravo. It appears that early applications of a strobilurin fungicide (two applied in July and one in August) were critical for maintaining good vine health, and contributed to the overall health of the stored product.

Fig. 1. 'Eva' tubers after 5 mo. storage, showing sunken lesions (L) compared with a healthy tuber (R). Similar damage appeared on 'Chieftain'. Fig. 2. Cut 'Eva' tuber from another treatment showing that the lesions were limited to the periderm and the inner cells of the cortex.