The Latest in Disease Control

November 6, 2003 - 14:09

What's new and not so new in plant disease control.

Now that I have the chance to explain to you what is new in
the disease control sphere, I find myself thinking that you might not have
taken advantage of what was new last year, or the year before last. The flower
industry is conservative about switching to new materials for disease
management. Sometimes this heel dragging stems from contentment, sometimes from
misinformation and sometimes from fear or frugality. But if you're reading this
article you are doing exactly what you ought to be doing: learning about what
is available and what is coming, so you are not caught without any knowledge of
options when a particular need arises. File this away so you can drag it out
the next time you want some additional guidance on an intriguing chemical.

Not Old Hat Yet

I'll start with the category of fungicides that are not old
hat yet . . . and that many of you haven't even tried. Strobilurins was
ironically named after a mushroom, Strobilurus tenacellus, which was the
original source of the chemical. Currently, we have three synthetic
strobilurins available to the flower industry: Compass-O (Olympic), Cygnus
(BASF) and Heritage (Syngenta). These are all considered
"reduced-risk" materials by the EPA. The strobilurins have an
excellent broad range of effectiveness against many common flower diseases,
particularly in the categories of leaf spots, anthracnose, scab, Botrytis,
downy mildew, powdery mildew and rusts. They can help with most of the fungi or
oomycetes (such as downy mildews) that spot plants. With spots caused by
bacteria, excessively low pH or furnace misbehavior, you'll need a different
solution.

The three strobilurin fungicides on the market now are
similar but not identical: If you are looking for primarily a powdery mildew
material, something for rusts or something for an anthracnose, you may prefer
one over another. Keep an eye on researchers' test results to help you with the
finesse of which strobilurin to use when. And pay attention to the labels. The
materials are not to be used repeatedly all season long -- you'll need to
rotate with materials that have complementary modes of action.

Strobilurins have the advantage of going beyond mere contact
action, but being systemic increases the likelihood of resistance developing in
the target population -- hence the strict usage instructions. Lest you think
that this kind of chemistry is only useful for foliar diseases, let me hasten
to add that trials of Heritage 50WDG in both New York and Florida are showing
extremely good control of Pythium and Rhizoctonia on impatiens at a low rate.

Another fungicide that is not new but still not well utilized
is Stature (SePRO). This product contains a mixture of dimethomorph and
mancozeb, and thus is ideally suited for use against downy mildews since it
contains two effective ingredients. Our trials with this material against the
three different downy mildews of pansy, mini-rose and snapdragon show it to be
an excellent protectant.

Still in the Pipeline

One new strobilurin from BASF is not available for
ornamentals use yet, but is showing a superb effectiveness against powdery
mildew in my trials. The test name is BAS-500. Additionally, a new,
still-being-tested formulation of Heritage is also looking very promising. Thus
you can anticipate that there will be even more effective strobilurins on the
market before long. But please note that the ones already available to you are
very good, broad-spectrum fungicides with low potential for phytotoxicity.
While we are talking about things not yet available, the combination of
mefenoxam and fludioxonil known as Hurricane continues to Á give good
results in trials of Pythium and Rhizoctonia root/stem base problems on
impatiens in both Florida and New York.

New Chemicals

Generic products have recently become available for
phosphites, iprodione, cholorothalonil, mancozeb and cupric hydroxide. They may
perform similarly to products with the same active ingredient, but they may
vary in phytotoxicity or residue or even effectiveness, so test them before
use.

Another new option in the powdery mildew department is
MilStop, a new potassium bicarbonate product marketed by BioWorks,
complementing its biological for root diseases, PlantShield HC. The initial
forté of MilStop will be powdery mildew, and its one-hour re-entry
interval (REI) gives it a special attractiveness for many operations.

One long-awaited new product for ornamentals and some
greenhouse vegetables, including cucumber and tomato, is Rhapsody 1.34% AS, a
biological control for foliar disease management (both bacterial and fungal)
from AgraQuest. The active ingredient in Rhapsody is the QST 713 strain of Bacillus
subtilis. You may already have an appreciation for Bacillus subtilis as the
active ingredient in Companion (Growth Products), which is a different strain
of the same species. There is a four-hour REI for Rhapsody. Rhapsody is also
labeled as a post-harvest dip on cut flowers. The information provided by
AgraQuest indicates that it has a multiple-site mode of action.

Trials at Chase Research Gardens have shown good benefit of
Rhapsody against bacterial diseases, including Pseudomonas leaf spots on delphinium
and impatiens and Xanthomonas leaf spots on geranium. Trials by David Norman at
the University of Florida-MREC have shown good powdery mildew suppression with
Rhapsody at 1 percent on gerbera. We have seen powdery mildew control on
mini-roses, verbena, petunia and poinsettia, as well as control of Botrytis on
poinsettias and geraniums, in trials at the Long Island Horticultural Research
& Extension Center. Chase Research Gardens has also noted some control of a
downy mildew and excellent control of the nasty Cercospora leaf spot on pansy
using Rhapsody. The label allows a 3- to 10-day treatment interval; most
studies have been conducted with a seven-day interval.

Biophos 43.07% L is a dipotassium phosphate product from
AgBio that is newly available for management of Phytophthora and other
diseases. It is labeled for drench at rates from 0.1-0.5 percent, and as a
spray at 1-2 percent. In our trials, a drench at 0.25 percent has completely
shut down Phytophthora drechsleri, which attacks at the stem base of
poinsettias. Biophos is similar in active ingredient and mode of action to the
familiar Phytophthora control Chipco Aliette WDG, which contains fosetyl-Al,
but it is likely to perform somewhat differently. Biophos and Aliette are of
special value in those cases in which the Phytophthora strain assaulting a crop
happens to be resistant to Subdue/Subdue MAXX (Syngenta). Biophos will be a
good choice to rotate with Subdue/Subdue MAXX for water mold root disease
management. It will also have other, yet-to-be-delineated control attributes,
because the phosphite type materials act by stimulating host plant defenses in
general.

The new Alude from Olympic is another new product in this
same category that has 45.8 percent phosphorous acid salts as its active
ingredient.

At the risk of repeating something that my entomological
counterpart might be simultaneously describing, I would like to point out a
disease-fighting use for the miticide Pylon 21.4% Á EC from Olympic,
which was recently registered for foliar nematode control on greenhouse
ornamentals. Foliar nematodes are occasionally seen on many greenhouse crops
and other herbaceous perennials in the nursery trade. When Vydate, Oxamyl and
Temik were lost to ornamentals producers due to their innate toxicity and
potential for groundwater contamination, foliar nematode symptoms began to be
increasingly common. Now Pylon, though developed as a miticide, has shown a
major side benefit in terms of foliar nematode control, and a supplemental
label has been written to legitimize this use. Nancy Rechcigl of Yoder
Brothers, Inc., reported applications made to Japanese anemone of Pylon at 4
oz. per 100 gal. at 14-day intervals were extremely effective at killing the
nematodes within the plants. Skillful use of this new chemical tool by
propagators should result in a marked improvement in the quality of crops prone
to foliar nematode within the next few years.

All in all, the industry should feel pleased at the recent
developments in ornamental plant health protection tools. There are new
solutions for the stubborn problem of foliar nematodes, a new biological
control with broad spectrum activity, some products that stimulate host plant
defenses and some reduced-risk chemistry that works against powdery mildews,
downy mildews and leaf spots. Given these new developments, it's time for me to
adjust my wish list!

Author's Note: With a subscription to Plant Management
Network, fungicide and nematicide test (F&N Tests) results for all
agricultural crops, including ornamentals can be found at
www.plantmanagementnetwork.org.

About The Author

Margery Daughtrey is senior extension associate in the Department of Plant Pathology at Cornell University, Long Island Horticultural Research & Extension Center, Riverhead, N.Y. She can be reached by phone at (631) 727-3595 or E-mail at mld9@cornell.edu.

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