Commercial Disease Controls
The Pennsylvania State University
Cooperative Extension

Watermelons

Seed Treatment

Check with seedsman to determine if seed has been treated with an insecticide and fungicide. If it has not been treated, use a mixture of thiram 75WP (1/2 teaspoon per pound, or 3 ounces per 100 pounds) and an approved commercially available insecticide

Disease Control

Damping-off
Apply the following in a 7-inch band after seeding. Use formula in the "Calibration for Changing from Broadcast to Band Application" section of Calibrating Granular Application Equipment to determine amount of Ridomil Gold or Ultra Flourish needed per acre.

mefenoxam -- l-2 pt Ridomil Gold 4E/A or 2-4 pt Ultra Flourish 2E/A

Bacterial Fruit Blotch
Obtain disease-free seed. Practice good sanitation during transplant production. Segregate different seed lots in the transplant house to reduce the chance of cross contamination. Use transplants from houses in which there were no seedling symptoms of the fruit blotch disease. Rotate to allow 2 years between watermelon plantings. Apply the following fungicide beginning at first bloom and repeat every 7 days:

copper, fixed--1.5 lb 53.8DF/A or OLF

Viruses (CMV, WMV, PRSV, and ZYMV)
Plant fields as far away from existing cucurbit plantings as possible to prevent aphid transmission of viruses from existing fields to new fields.

Ozone Injury
Ozone will cause chlorosis and upper surface bronzing and scorching on the older leaves, which leads to defoliation. Sugar Baby is one of the more sensitive varieties.

Fusarium Wilt
Use a long rotation of at least 5 years. Use resistant varieties when possible. The varieties 'Seedless Sangria' and 'Revolution' are the only seedless variety that have been tested in the region that is tolerant to Fusarium wilt resulting from race 2. Royal Majesty is a seeded variety with tolerance to race 2. Do not plant other watermelon varieties in fields where Race 2 is present.

Anthracnose
Excellent resistance is available in some varieties. Use resistant varieties when possible. Begin fungicide applications when vines run or earlier if symptoms are detected.

Alternate:
chlorothalonil (Bravo,Echo, Equus) -- 2-3 pt 6F/A or OLF (Use low rate early in season.), plus Topsin M--0.5 lb 70WP/A
with:
Pristine 18.5 oz 38WS/A, or
a tank mix containing chlorothalonil plus one of the following every 14 days:

azoxystrobin (Amistar--3.5-5 oz 80WDG/A or Quadris--11 - 15.4 oz 2.08F/A), or
Cabrio--12-16 oz. 20EG/A, or
Tanos--8 oz 50 WDG/A (must be tank mixed with either chlorothalonil, mancozeb, or copper).

If resistance to QoI fungicides exists in the area, do not use Quadris, Amistar or Cabrio. Use a fungicide from a different fungicide class.

Downy Mildew
The disease generally does not occur until mid-August, however, scout fields for disease incidence. Refer to the Cucurbit Downy Mildew Forecasting website (http://www.ces.ncsu.edu/depts/pp/cucurbit/) for current status of the disease. Begin sprays when vines run, or if disease occurrence is predicted for the region. Preventative applications are much more effective than applications made after disease is detected. Resistance to the QoI/Class II fungicides has been reported in the Eastern U.S. The following are the most effective materials (tank mix these products with a protectant such as chlorothalonil (Bravo, Echo, Equus)--1.5 pt 6F/A or OLF or maneb--2-3 pt 4F/A or OLF):

Ranman--2.1-2.75 fl oz 400SC/A or
Previcur Flex--1.2pt 6F/A or
Curzate--3.2 oz 60DF/A or
Pristine--12.5-18.5 oz 38WG/A, or
Tanos--8 oz 50 WDG/A or
Cabrio--8-12 oz. 20WG/A.

Materials with different modes of action (FRAC group) should be alternated.

Sprays should be applied on a 7-day schedule. Cabrio, Pristine and Tanos should not be alternated with each other or used consecutively. Under severe disease conditions spray interval may be reduced if label allows.

Alternaria Leaf Blight
Begin sprays when vines begin to run.

Alternate:
chlorothalonil (Bravo,Echo, Equus) -- 2-3 pt 6F/A or OLF (Use low rate early in season.), plus Topsin M--0.5 lb 70WP/A with:
Pristine 18.5 oz 38WS/A, or
a tank mix containing chlorothalonil plus one of the following every 14 days:
azoxystrobin (Quadris--11 - 15.4 oz 2.08F/A or Amistar--3.7-5 oz 80WDG/A or ), or
Cabrio--12-16 oz. 20EG/A

If resistance to QoI fungicides exists in the area, do not use Cabrio, Pristine, Amistar, or Quadris. Use a fungicide from a different fungicide class.

Gummy Stem Blight
Occurs primarily in late summer. Begin sprays when vines begin to run.
Fungicides at risk for resistance development such as strobilurins/Class 11/QoI fungicides (Cabrio, Flint, Amistar, and Quadris ) should be tank-mixed with a different class of fungicide (see Table E-8) to delay onset of resistance. When tank-mixing use at least the minimum labeled rate of each fungicide in the tank mix. Alternate with a non-QoI fungicide effective against gummy stem blight. Do not apply a QoI fungicide more than 4 times per season.

Alternate:
chlorothalonil (Bravo, Echo, Equus) -- 2-3 pt 6F/A, or OLF
with a tank mix containing chlorothalonil and
Pristine 12.5-18.5 oz 38WS/A, or
azoxystrobin (Quadris--11 - 15.4 oz 2.08F/A or Amistar--3.7-5 oz 80WDG/A or ), or
Cabrio--12-16 oz. 20EG/A

If resistance to Quadris or Amistar exists in the area, do not use Cabrio, Flint, Amistar or Quadris. Use a fungicide from a different fungicide class.

Phytophthora Blight
Fields should be adequately drained to ensure that soil water does not accumulate around the base of the plants. In addition, when the vines begin to run, subsoil between rows to allow for faster drainage following rainfall. When conditions favor disease development, apply the following:

Forum--6 oz 4.18SC/A, (must be tank mixed with another fungicide active against Phytophthora blight on watermelon such as fixed copper), or
Gavel--1.5-2 lb75DF/A
Tanos--8 oz 50 WDG/A (suppression only), or
Ranman--2.75 fl oz 400SC/A

Powdery Mildew
This disease was observed for the past few seasons in Delaware and Maryland and could occur in other states. Detection of powdery mildew is more difficult in watermelons than other cucurbits because sporulation is sparse and masked by leaf color. Look for chlorotic spots on the upper leaves, and then inspect corresponding lower leaf surface with a hand lens to confirm presence of the fungus.

The fungus that causes powdery mildew has developed resistance to several classes of fungicides. The fungicides at risk for resistance development such as Strains of the pathogen that are highly resistant to some strobilurins/Class 11/QoI fungicides (Cabrio, Flint, Amistar, Quadris and Tanos) have been reported in the Eastern U.S. in 2004 and 2005.

Powdery Mildew generally occurs from mid-July until the end of the season. Excellent resistance is available in all recommended cucumber varieties. Observe fields for the presence of powdery mildew. If one lesion is found on the underside of 45 old leaves, begin the following fungicide program:

Alternate:
Chlorothalonil (Bravo, Echo, Equus)--2-3pt 6F/A or OLF plus Nova--5oz 40W/A, or
Chlorothalonil (Bravo, Echo, Equus)--2-3pt 6F/A or OLF plus Procure--4-8 oz 50WS/A, with

Pristine--12.5-18.5 oz 38WG/A

Note: Where trade names are used, no discrimination is intended and no endorsement by the Cooperative Extension Service is implied.
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.