Guidelines for Managing Cucurbit Powdery Mildew in 2006
There are new varieties with resistance to powdery mildew and
several fungicides for
managing this the most important cucurbit disease occurring every year where
these crops are grown. Fungicides and resistant varieties are the
only management tools. In NY, Procure was
recently registered for this use and Quintec was granted a specific exemption
under Section 18 of FIFRA. Below are recommendations for 2006 followed by
information about these, then separate articles on Procure and Quintec.
- Select varieties
with resistance when possible. Lists of resistant varieties are at http://vegetablemdonline.ppath.cornell.edu/Tables/TableList.htm.
- Start
fungicide applications when symptoms are just beginning to develop (IPM)
or when fruit are starting to enlarge (preventive). The IPM action threshold is 1 of 50 older
leaves with symptoms. Look at the lower
as well as upper surface of leaves when scouting. Use a 14-day interval
with resistant varieties; 7-day with other varieties.
Preventive (optional). Protectant alone
before symptoms develop.
Week 1. Quintec (FRAC Group 13) plus
protectant for non-edible peel cucurbits.
Week 2. Procure or Nova (FRAC Group 3) at
high label rate plus protectant.
Week 3. Pristine (combination
of FRAC Group 7 and 11) plus protectant.
For edible peel cucurbits (summer squash
and cucumber), and in states where Quintec is not available, use a two-way
alternation without Quintec.
Continue alternating for as long as good
powdery mildew control is needed in the crop.
Use protectant alone if powdery mildew is
not being controlled well on the lower leaf surface and near the end of the
season when level of control needed does not justify the expense of including a
mobile fungicide.
The fungicide program recommended for 2006 was selected to manage
fungicide resistance as well as ensure effective control especially if
resistant strains able to tolerate the fungicide rates used are present. Managing resistance is a critical aspect of
effectively controlling cucurbit powdery mildew because this pathogen has a
history of control failure due to developing resistance. Tactics need to be implemented beginning when
a new product is first used because the main goal of resistance management is
delaying its development rather than managing resistant strains once present. General tactics are alternating among
products at risk for resistance, which are the mobile fungicides due to their
single-site mode of action, and tank-mixing with protectant fungicides which
have low resistance risk. Mobile fungicides
are needed to control powdery mildew on the lower leaf surface.
Scout
weekly for symptoms by examining both surfaces of at least 50 older leaves
beginning at fruit set and continuing until powdery mildew is detected. Then immediately begin applying fungicides on
a 7-day schedule (14-day with resistant varieties). A critical component of a fungicide
resistance management program is not applying at-risk fungicides when disease
is established (eradicant or curative application); therefore, where it is not
feasible to implement a scouting program, use a preventive application
schedule. Evaluate control during the
growing season, especially on lower leaf surfaces. Report poor control promptly to extension
specialists so the possibility of fungicide resistance being the cause can be
investigated. Stop using mobile fungicides if powdery
mildew is becoming severe; use only protectant fungicides. Applying fungicides at-risk for resistance
when disease is severe creates a lot of pressure for selecting resistant
strains. While spores of the powdery
mildew fungus overwintering in soil in the northeast are generally considered
an inconsequential source of inoculum for epidemics the following year given
the quantity of wind-dispersed spores from southern production areas, just a
few spores of a new resistant strain could be very important.
Quintec, Procure and Nova have narrow
spectrum activity. Therefore it is
critical to monitor crops for diseases other than powdery mildew. When other diseases are a concern, use
broad-spectrum protectant fungicides like chlorothalonil and copper rather than
sulfur or oil. Pristine contains a QoI (strobilurin)
fungicide and thus has broader activity than the other mobile fungicides for
powdery mildew. Quintec is available for
use in NY under an emergency exemption (Section 18). Users must have a
copy of this label when they use this product, which can be obtained from local
distributors and extension office as well as at http://pmep.cce.cornell.edu/regulation/sec18/2006/index.html.
Approximate
cost of the fungicides in this program are $16.89 for Quintec at 4 oz/A, $17.63-$23.50
for Procure at 6-8 oz/A, $21.53 for Nova at 5 oz/A, and $30.73-45.47 for Pristine
at 12.5-18.5 oz/A.
Three-way alternation
programs with Quintec, Pristine and Nova started after reaching the IPM
threshold with a total of 5 weekly applications applied to pumpkin provided
excellent control of powdery mildew on upper leaf surfaces (96-97%) and lower (71-74%)
when evaluated in 2005 in Riverhead. The
protectant included each week was sulfur (4 lb/A Microthiol Disperss). The QoI component of Pristine did not appear
to be contributing to powdery mildew control because Pristine tested alone was
not more effective than Endura, a formulated product not registered for use on
cucurbits with the active ingredient boscalid, which is the other active
ingredient in Pristine. For another
treatment sulfur was applied twice on a preventive schedule before initiating
the fungicide program at the IPM threshold. These two applications did not delay when threshold was reached which
was one objective of this treatment. Control achieved with this treatment was numerically better (98% and 87%
control) but not significantly better than the similar treatment without
preventive sprays of sulfur. Greater
benefit may be achieved when an entire planting is treated.
Please Note:
The specific directions on fungicide labels must be adhered to -- they
supersede these recommendations, if there is a conflict. Any reference to commercial products, trade
or brand names is for information only; no endorsement is intended.
SEE ALSO: Successful control of
powdery mildew in cucurbit crops challenged by Evolving pathogen (May 2008)
