August Disease Reminders for Commercial Producers in PA
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A. A. MacNab, Plant Pathology Dept., Penn State University
A. DISEASE IDENTIFICATION
B. ASPARAGUS
C. BEANS
D. BEANS, LIMAS
E. BEETS
F. CABBAGE:
G. CARROTS and CELERY
H. CUCURBITS
I. EGGPLANT
J. ONIONS
K. PEPPER
L. PUMPKIN and SQUASH
M. SWEET CORN
N. TOMATOES
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A. DISEASE IDENTIFICATION
Identification information is available in the colored publication, "Identifying
Vegetable Diseases" which is available from most Pennsylvania Extension offices, as
well as from Penn State University.
B. ASPARAGUS
Rust: Continue applying fungicide sprays to young plantings. In addition to
mancozeb and Bravo, Nova is now labelled for rust control on asparagus.
C. BEANS
Mosaic Viruses: Use resistance to BV-1. Provide good weed control; weeds are a
source of bean viruses. Do not plant near clovers; they are a source of bean viruses. Do
not make successive plantings in adjacent strips or fields; the few diseased plants
that appear in early plantings act as an important virus source for later adjacent
plantings.
White Mold and Gray Mold: Apply protective fungicide sprays when warranted. Wet
conditions immediately before and during bloom promote disease development. Ronilan
is very effective, although only labelled for snap beans; Endura, Rovral and Topsin M
also are good when timed well, and can be used on snap and other beans.
D. BEANS, LIMAS
Downy Mildew: During wet weather, when conditions favor disease, use fungicides.
Conditions that favor late blight of tomatoes and potatoes also favor downy mildew of
lima beans.
E. BEETS
Leaf Spots: Use fungicides where disease usually occurs.
F. CABBAGE:
Fusarium Yellows: This disease is favored by hot weather. When possible, avoid
susceptible varieties.
Clubroot: Where present, it is too late to apply controls for this year. Determine
the source if possible. Then plan rotation, pH adjustment, or Terraclor use for next
season. Wet soil conditions favor development.
Downy Mildew and Leaf Spots: These diseases become most important late in the
season. Where anticipated and warranted, fungicides provide some control. For downy
mildew, Ridomil Gold/Bravo 81W (1.5 to 2 lb/A) is effective and can be applied at
14-day intervals until 7 days before harvest. Weekly applications of Bravo and maneb
also provide control. For downy mildew only, Aliette can be used (3 to 5 lb/A) at 14
day intervals until 3 days before harvest. In addition, Amistar/Quadris can be used for
Alternaria leaf spot control but is not labelled for downy mildew control.
G. CARROTS and CELERY
Leaf Spots: Continue regular fungicide sprays; they are most important for the
remainder of the season. Wet weather will promote their appearance and development.
H. CUCURBITS
Bacterial Wilt: Where present, it is too late to attain control this year. Plan
cucumber beetle (bacteria carrier) control for next year.
Powdery Mildew, Leaf Spots, and Blights: Use regular fungicide sprays unless
varieties are resistant to the diseases of concern. Leaf diseases are of major
importance on muskmelons (cantaloups) during late season. Fungicides are outlined
in the Vegetable Production Guide. Follow the resistance management program
closely to minimize the risk of the powdery mildew fungus developing resistance to
fungicides. Ridomil/Bravo 81W is especially good for downy mildew, a disease that
appears less frequently than powdery mildew in Pennsylvania.
Scab: For susceptible varieties, use fungicides when conditions are cool and wet.
For cucumbers, use resistant varieties for next year. Beware of this disease if late
plantings of susceptible cucumbers are planted.
Mosaic Viruses: Use resistant varieties. When resistant varieties are not available,
control perennial weeds, plant in large fields, and control aphids.
Fusarium Wilt: It is too late to apply controls for this year. For future years, try
rotation, resistance when available, and if necessary, soil fumigation. Symptoms
appear first on plants in wettest areas of fields, and the disease is more prevalent in
cool soil than in warm soil. The variety Athena may have the most resistance.
I. EGGPLANT
Verticillium Wilt: Follow rotations that avoid susceptible crops for as many years
as possible. Where present yearly, consider fumigation where crop value warrants the
expense.
J. ONIONS
Leaf Spots: Apply fungicides on a regular schedule, especially for those planned for
storage. Defoliation diseases are active in August. Note that Ridomil Gold/Bravo,
Ridomil Gold MZ, and Aliette are labeled for onions, and are materials of choice if one
has difficulty controlling downy mildew. Other materials listed in the Commercial
Vegetable Production Guide are good for other leaf diseases that affect onions.
K. PEPPER
Mosaics and Virus Spots: It is too late to affect control for this year. For future
years, use TMV resistance, plant in large fields, control aphid vectors, and eliminate
perennial weeds near fields.
Bacterial Spot: Where present, determine source. Did it come on plants? If present,
start basic copper sprays early and tank mix with maneb. Plan to rotate to fields not
recently planted to tomatoes or peppers. This can be a serious problem. Inoculum can
be seed-borne. If disease is detected in a field, do not work in the field when plants
are wet. The bacteria that cause this disease are spread less when plants are dry than
when plants are wet.
Phytophthora blight: This disease is promoted by wet soil (poor drainage and/or
heavy rainfall). The most severe losses are reported on cherry and cheese types.
Follow a 3-year rotation between susceptible crops such as pepper, cucurbits,
eggplant, and tomato. Ridomil Gold 2E (1 pt/A) or Ultra Flourish 2E (1 qt/A) soil
applications (banded over the row at planting, and directed onto the soil at the base of
plants at 30 days and 60 days after planting) are helpful. In addition, provide the best
drainage possible; in some areas, for susceptible peppers, it is necessary to grow them
on high ridges. In place of a "60 days after planting" application (above), Ridomil
Gold/Copper can be applied as sprays at 10 to 14-day intervals to prevent the stem and
fruit rot stage of this disease. If the "60 days after planting" application is made,
fixed copper alone can be applied as a spray at 7- to 10-day intervals to control the
stem and fruit rot stage of this disease. As of June, 2004, Acrobat has a federal label
for suppression of Phytophthora blight.
L. PUMPKIN and SQUASH
Powdery Mildew: Use regular fungicide sprays. Start applications no later than
when powdery mildew is detected on 1 leaf in 50 leaves; usually, powdery mildew
symptoms appear first on the underside of leaves. Fungicides are outlined
in the Vegetable Production Guide. Follow the resistance management program
closely to minimize the risk of the powdery mildew fungus developing resistance to
fungicides. Ridomil/Bravo 81W is especially good for downy mildew, a disease that
appears less frequently than powdery mildew in Pennsylvania.
Phytophthora blight: This disease is promoted by wet soil (poor drainage and/or
heavy rainfall). Follow a 3-year rotation between susceptible crops such as pepper,
cucurbits, eggplant, and tomato. Ridomil Gold 2E (1 to 2 pints/treated A) applied at
planting as labelled for Pythium and cottony leek control may be helpful. Foliar
applications of Acrobat tank mixed with a fixed copper material may be helpful.
M. SWEET CORN
Leaf Spots and Rust: Some varieties have some resistance. Fungicides are effective
for leaf spots but less effective for rust. Quadris, Tilt and mancozeb are labelled for
rust; Quadris and Tilt are somewhat systemic and are the materials of choice when rust
is especially difficult to control at the end of the season.
Maize Dwarf Mosaic (MDM): Aphid vector control may help in large fields. Where
MDM is prersent, plan to try tolerant varieties in future years.
N. TOMATOES
Bacterial Speck and Spot: If spots are a yearly problem, and symptoms appear on
leaves, continue sprays with basic copper plus maneb/mancozeb. Next year, rotate to
new fields, use pathogen-free seed, and spray seedlings regularly with streptomycin
before transplanting. If symptoms are not present on leaves now, the fixed copper
applications may not be needed for the rest of the season. Where present, try to avoid
working and spraying in plantings when they are wet.
Bacterial Canker: Bacterial canker is appearing in some fields. I suspect that
inoculum source is either infected seed, or infested soil. The causal bacteria can
persist in soil for at least 3 years! Control is very difficult at this time. Do not work
in affected plantings when plants are wet. Application of fixed copper tank mixed
with either maneb or mancozeb, as for bacterial spot and bacterial speck, may help
slow spread in fields.
Fruit Rots and Leaf Spots: Where possible, rotate and provide adequate fertility.
Continue a good fungicide program. In addition to the standard fungicides, Bravo and
mancozeb, which provide good control, new strobilurin fungicides (Quadris, Flint, and
Cabrio) are excellent new fungicides for early blight control. The strobilurins should
be alternated with a protectant material.
Late Blight: Fungicides are needed to protect tomatoes (and potatoes) whenever
environmental conditions favor disease development. As listed in the Vegetable Guide,
Tanos, Acrobat, and Previcur Flex are three fungicides with at least translaminar
systemic activity for control of late blight on tomatoes. Up-to-date information is
available on the toll free "Hotline": 1-800-PENN-IPM = 1-800-736-6476.
A. A. MacNab, Prof. Plant Pathology, Penn State University
Updated: July 1, 2005
Information provided is intended for consideration by the user, but is not intended to
be a recommendation. Production decisions should be based on consideration of many
types of information (scientific, experiential, economic, legal, etc.) available to the
user. Where trade names are used no discrimination is intended, and no endorsement by the Cooperative Extension Service is implied.