Disease Diagnosis
Many resources are available through cooperative extension to alert farmers of
impending disease problems. A website developed and run by cooperative extension
faculty from North Carolina State University forecasts the initial outbreak of cucurbit
downy mildew at locations throughout the east coast (http://cdm.ipmpipe.org/). Farmers
who grow pumpkin, squash, watermelon, or other cucurbits can use it for an early
warning of downy mildew outbreaks. In the mid-Atlantic region, watermelon and
cantaloupe (muskmelon) growers should visit the cooperative extension site hosted at
the University of Maryland, where MELCAST disease forecasting is provided.
MELCAST http://mdvegdisease.umd.edu/forecasting/index.cfm provides information on
how conducive the weather is for development of Alternaria leaf blight and gummy stem
blight. The MELCAST model was developed to schedule conventional fungicide
applications, but organic growers can learn if weather is favorable or unfavorable for
these diseases and adjust their biofungicide sprays, or their harvest practices
accordingly.
8
Accurate and timely diagnosis of vegetable diseases is the most important step for
successful implementation of effective disease management practices. Although many
disease symptoms can be difficult to distinguish, there are numerous resources
available in print and online to aid the diagnosis process. When confronted with an
unknown plant problem, the most reliable method to identify the cause is to contact an
expert. Local county extension educators are your first line of defense; they often have
seen similar problems and they may be able to provide a presumptive diagnosis. As
soon as you see a problem that needs diagnosis, call your county extension educator.
He or she will request a digital image, ask you to drop
off a sample, or visit your field. If the extension educator
isn‟t able to identify the problem, they will consult with a
specialist. The extension educator or specialist will
search for signs or symptoms of disease. Disease signs
are the observed presence of a pathogen, such as
sclerotia, spores, or spore producing bodies (Figure 2).
Symptoms are the observable response of the plant to
disease (e.g. wilt, leaf spot, or vascular browning). The
extension educator or specialist will make a presumptive
diagnosis if the symptoms are entirely consistent with
the pathogen observed and if no other possible cause is
present. If the cause still remains unknown, then a
sample of the infected plant should be sent to the University of Maryland‟s Plant
Diagnostic Laboratory for further evaluation (www.plantclinic.umd.edu