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Mid-Summer Report
Card on the Occurrence and Affects of Late Blight on Tomato and Potato in
New York, 2009
T. A. Zitter
Plant Pathology and Plant-Microbe Biology Section
Cornell
University
Ithaca,
NY
14853
August 19, 2009
Although admittedly dreading a trip to eastern NY last week (Aug 10-13)
for fear of seeing one dead tomato or potato field after another, I was
pleasantly surprised to see that not every grower (conventional or organic) had
experienced a complete wipeout of their crops. To be sure, some sporulating late blight was found in many fields
examined at each stop, still it was apparent that the disease, although
wide-spread, was not causing total destruction either, and that growers had
taken the appropriate steps to minimize their losses. To be sure, this was at the expense of many
fungicide applications and a lot of time and sweat expended to control the
disease, but the results were gratifying to see. Homeowner gardeners and smaller organic
operations suffered the greatest losses, and they will be glad to garden
another season in the absence of late blight.
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Why an endemic problem but not an epidemic catastrophe in susceptible crops this summer?
As
previously documented, the spring and early summer of 2009 was setup for losses
in tomato and potato crops of unprecedented proportions, given that infected
tomato plants had been sold to unsuspecting home gardeners from large “big box”
stores and environmental conditions (frequent rains and cool temperatures) were
common throughout the NE. Additionally, in the case of potatoes, late
blight occurrence in 2008 in some seed producing states, also could provide inoculum on infected tubers used for the 2009 crop. Based upon tests to identify the clonal
lineages of Phytophthora infestans conducted in Dr Fry’s lab at Cornell, the isolates recovered from infected
tomato and subsequently infected potato did not fit any of the previous late
blight genotypes previously identified in NY. The new genotype is mating type A2 (perhaps P-T, but not as virulent as
some in the past), unlike the previous genotypes like US-11 (P-T) and US-17 (p by
laboratory tests only-T) which are A1 mating types and were even more
destructive on tomato. The pathogenicity
indicated by (P-T) refers to pathogenic specialization on both crops, where capital P and T indicates primary pathogens of both crops, while a lower case letter indicates less pathogenicity for the crops. Separate infections did occur in potato in
western NY and were identified as US-8 (mating type A2, P). US-8 is now the primary genotype
of late blight infecting potato on a regional basis. So, our salvation this summer was that the
primary genotype(s) spread throughout the region were apparently not as
pathogenic on tomato and potato as we have encountered in other years. This is little consolation to homeowners and
organic growers who suffered total loses, but does explain how some growers
were able to keep losses to a minimum.
What steps worked for growers faced with
late blight in 2009?
I will divide this discussion into
two parts, since we are dealing with the two different genotypes in NYS, and
the actions taken are different. In the
case of the unidentified A2 type originating initial from tomato (not quite
P-T, but close) that spread to tomato and potato, growers (both conventional
and organic) relied upon a tight fungicide program (5-7 day schedule) with late
blight specific fungicides in conventional operations (products including
contact materials like chlorothalonil, Gavel, and Ranman, and translaminar
materials like Curzate, Previcur Flex, Revus, and Tanos). Organic growers relied on copper fungicides
applied on a 3-4 day schedule (Nu-Cop and Basic Copper 53). Another procedure followed, especially by
organic growers, was to flame out of the most aggressive hot spot areas located
near tree lines to remove the most heavily infected plants early in the initial
spread of late blight. This practice
undoubtedly saved a lot of the crop and allowed copper sprays a chance to
reduce remaining infections. In organic
operations we also saw growers cutting down infected potato foliage of more
susceptible varieties in an effort to reduce the inoculum level in their
fields.
In the case of commercial potato
fields faced with the need to control US-8 (mating type A2, P) within their
crop (likely originating from infected seed tubers), the steps take were more
aggressive. Hot spots of infection in
fields were killed as soon as detected and then an aggressive 4-5 day spray
schedule was followed using the late blight specific products mentioned above.
The bottom line in both cases this
season, the successfully control on late blight hinged on the application of
appropriate fungicides on a very tight schedule.
What can we expect for the remainder of the
season?
Unfortunately, we are not out of the
woods by any means, as what I call “back filling” of infections is still
occurring in remaining susceptible crops (potato and tomato) in both homeowner
gardens and commercial acreages (both organic and conventional). Reports of late blight infections just now
appearing in home gardens and isolated cropping areas are just now coming
in. Although a brief spell of hot
weather may have slowed down late blight a bit, it is still sporulating and
producing inoculum during our heavy evening dew periods. All concerned interests must continue on a
regular fungicide program until the crops are finished. Special care must be made in the case of
potato to examine harvested tubers to make sure they are free of tuber
blight. It is advisable to move the crop
as soon as possible to reduce tuber infections in storage. The other significant reminder is to make
sure that all tubers are harvested and that special attention is directed to
removing and destroying volunteers that may survive the winter season and have
the potential of carrying over late blight to next year. A common problem is encountered when corn is
planted as the rotational crop in fields that had late blight this season. Volunteer potatoes are difficult to rogue out
when hidden by the emerging corn or other rotational crop.
What can we learn from tomato and potato
crops with more limited infections?
Differences in the amount of
infection of potato and tomato, both traditional varieties and heirlooms, have
been observed this year across the state. For sure there are definitely differences in the amount of infection for
both crops. However, given that the
predominate clonal lineage that occurred this year is apparently not as
aggressive as those encountered recently, this might lead one to consider a
variety as resistant or tolerant to late blight, when in fact in another year
is could perform differently. One fact
does remain, with the exception of comments made about overwintering of
inoculum on potato tubers, next year we begin the season with a “clean slate”,
one that will be more kind to all fanciers of tomato and potato.