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Experiences with Late Blight in New
The Tomato Magazine
By Dr. Thomas A. Zitter
Late blight of tomato and potato caused by Phytophthora infestans occurred
in several states in the Northeast and Mid-Atlantic region during the
2003 season, and it has cropped up again in 2004. In potato, late blight
was first reported in 2003 from western Pennsylvania (Crawford and Erie
counties) on July 24, from Maine, New Jersey and Maryland in mid-August,
and in three counties in western New York in late August. The isolates
from New York were identified as the predominant potato genotype in the
United States, US-8 (mating type 2), which is metalaxyl resistant.
In tomato, late blight was first found in commercial fields in Lancaster
County, Pa., and in processing tomato fields in Cumberland County, N.J.,
in early August. Subsequent spread in New Jersey occurred in both commercial
fresh market tomatoes and home garden locations from late August and into
September. The tomato isolates from Cumberland County were assigned to
an unknown lineage with an A2 mating type. From all accounts, the tomato
late blight isolates were more aggressive on tomato than the US-8 isolates
were on potato.
Since the early 1980s, late blight has become an increasing problem for
potato and tomato growers around the globe. New and aggressive strains
that originated in Mexico have displaced the previous indigenous strains.
These exotic strains were first detected in Europe, but have subsequently
been found around the world, including the U.S. Several independent migrations
have occurred such that the migrant populations in North America are currently
very different form those in Europe. In fact, the populations that exist
in Western Europe are so diverse and aggressive that the populations can
be viewed almost as a "second center of origin" after central
Mexico, and represents a major crisis in many of these countries. The
exotic strains that occur in the U.S. also came from Mexico, but from
a different location and contain some different characteristics. The exotic
strains that were introduced into the U.S. were primarily of single genotypes,
so that the structure of P. infestans populations in the U.S. remains
very simple, with only a few genotypes with widespread distribution (i.e.,
US-7, US-8, etc.). As in Europe, these exotic strains have displaced the
previous indigenous strains (US-1, US-6).
Why did the exotic strains displace the previous indigenous strains and
what is the significance of the potential for sexual recombination? P.
infestans has two mating types, A1 and A2, which can lead to sexual recombination
and the production of oospores. This provides a mechanism for eliminating
deleterious mutations that can accumulate in an asexual lineage. The previous
indigenous strains like US-1 are thought to have been asexual for 150
years. Sexual reproduction provides the mechanism for producing particularly
fit genotypes, and this may include providing resistant to some fungicides
such as metalaxyl and mefenoxam, Ridomil and Ridomil Gold, respectively.
New strains may also require significantly more fungicide (up to 25 percent
more) for adequate suppression of late blight. Furthermore, the ability
of P. infestans to produce the sexual spore form (oospore) would provide
a mechanism for this organism to overwinter in the absence of the host.
Sporangia produced by asexual reproduction may survive in the soil for
days or weeks while oospores produced by sexual reproduction may survive
in the soil for months or years. There is circumstantial evidence that
US-11 lineage found on tomato and potato in New York was generated by
sexual reproduction in the western U.S. The long distance movement of
sporangia on weather fronts is thought to be 10 to 50 miles, but the former
Russian KGB, when studying the use of late blight as a biological agent,
felt they could spread the pathogen over 1,000 km (600 miles).
The occurrence of new exotic and more virulent strains of P. infestans
has also forced plant pathologists to reexamine the biology of this organism.
The host range has been extended to include hairy nightshade (Solanum
sarachioides), garden petunia (Petunia hybrida), and bittersweet (S. dulcamara).
The spread of late blight from petunia to tomato transplants is a real
Fortunately our situation in North America is not as bleak as in Holland
where the potato crop may require as many as 22 applications, with applications
beginning as early as April. Other than late blight resistance for metalaxyl
and mefenoxam, there are no fungicide resistance concerns with products
currently used to control late blight, but most products are tank mixed
with protectant fungicides and materials with different modes of action
are alternated. In the case of early blight control in potato (Alternaria
solani) and in tomato (A. tomatophila), there is reduced sensitivity to
Quadris (azoxystrobin) due to a mutation of the F129L location in the
cytochrome b protein sequence. This reduced sensitivity should also apply
to the other members of the Group 11 fungicides, such as other strobilurins
like Headline and Gem, or the newer Qol fungides with the same mode of
action, Tanos and Reason. Consequently, these products with multi-site
fungicides like chlorothalonil or mancozeb or use of a premix of these
fungicides (Quadris Opti is a mixture of azoxystrobin and chlorothalonil).
Growers should limit the use of Group 11 fungicides to one-third of their
total spray program.
Tomato Fungicide Trial
A fungicide trial was conducted at Freeville, N.Y., in 2003, using the
variety Supersonic. Disease spreader rows were inoculated with early blight
(Alternaria tomatophila (light phenotype) on July 25, and with late blight
(Phytophthora infestans US-17) on Aug. 15. Heavy disease pressure occurred
for both diseases, encouraged by frequent rains and supplemented with
overhead irrigation. Fungicides were applied with a CO2 pressurized boom
sprayer at 60 psi, delivering 28.8 gal/A through four TeeJet XR11003 flat
fan nozzles spaced 20 inches apart. Sprays were applied on a 7-day schedule.
Fungicides specific for late blight in addition to the regular products
(Bravo, Quadris, Amistar, Quadris Opti and Cabrio) included Ranman (cyaxofamid),
Reason (fenamidone) and Tanos (famoxadone + cymoxanil). The initial sprays
were used for early blight control, while sprays scheduled for the balance
were used to control both early blight and late blight. The late blight
specific materials were applied for the first time on Aug. 13, two days
before late blight was introduced into the spreader rows.
All fungicide treatments provided good to excellent control of early blight.
For example, the control showed 50-75 percent defoliation compared to
treatment 8 which included Quadris Opti with a rating of 3 percent infection.
Similarly, all fungicide treatments provided good control on tomato foliage
for late blight, and all were significantly better than the control. As
is typical of the new exotic strains, US-17 was particular severe for
tomato fruit infection. There were no significant differences among the
better late blight materials tested, with exceptional control achieved
with Ranman, Reason and Tanos used in alternation with protectant fungicides.
Bravo, when used preventatively for late blight control, also performed
well. Treatment 15, consisting of Phostrol used alone and mixed with Dithane,
provided good foliar control for late blight, but provided less control
later in the season, as indicated by the higher number of fruit infections.
Providing only two weekly sprays over a critical 12-day period did not
provide the level of protection achieved by the other systemic materials.
During periods of intensive disease pressure, Phostrol mixed with a protectant
(both at the highest rates) and on a 5-day schedule would have provided
Fry, W.E., and Goodwin, S.B. 1997. Re-emergence of potato and tomato late
blight in the United States. Plant Diseases 81:1349-1357.
Fry.W.E., and Smart, C.D. 1999. The return of Phytophthora infestans,
a potato pathogen that just won't quit. Potato Research 42:279-282.
© 2004 Columbia Publishing
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