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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.
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| 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. |
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