Diseases of watermelon may reduce yield by increasing respiration, or reducing photosynthesis, nutrient uptake or translocation, and therefore result in smaller or fewer or no fruit. Reduced photosynthesis also may reduce sugar accumulation in fruit reducing fruit quality. Pathogens also may directly invade fruit or damage foliage to the extent that fruit become sunburned and therefore unmarketable. Disease management includes site selection: selection of disease resistant cultivars, disease free transplant production, protecting seedlings and growing plants once they are in the field, and sanitation after harvest.
Pathogens may survive between crops in host tissue as a special survival structure or on a living cucurbit host. Therefore diseases require different crop free (rotation) periods to minimize initial inoculum and disease development.. Transplants should be grown using seed that has been tested and found to be symptomless (watermelon growers will need to sign a waiver in order to purchase seed)., under conditions that minimize disease development. Cultural controls are extremely important in the greenhouse because chemical control options are limited.
After the transplants go to the field, application of fungicides becomes the major disease control option. There is mounting evidence that spraying according to a weather based fungicide application model , such as MELCAST, using appropriate threshold values will minimize fungicide costs while controlling disease and maintaining yield. However, disease models should always be tested locally prior to implementation.
Harvest often occurs while fungicides are still being used. After harvest, crop refuse should be plowed into the ground to speed decomposition of the plant material where many pathogens survive. Once the refuse decomposes, some pathogens cannot survive.
DAMPING-OFF
Damping-off refers to a disease caused by a number of fungal disease organisms that cause seeds to rot before they germinate, shoots to decay before they emerge, or seedlings to collapse after emergence. Disease severity depended on presence of pathogen, inoclum levels and the weather. Fungicide seed treatments are often used to manage damping-off.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens
Captan (North Carolina)
Widely used as a seed treatment for a wide range of fungi
IPM and/or resistance management issues: None
Regional differences: Should not be any
4. Other chemical pesticides
Ridomil, Gold EC (mefenoxam)
Used as application after transplanting for Pythium damping off.
Apron (metalaxyl)
seed treatment for Pythium damping-off
Maxim (fludioxinil)
seed treatment for Fusarium and Rhizoctonia damping-off.
Thiram (North Carolina)
Old standby, used extensively for seedling damping-off
IPM and/or resistance management issues: None
Environmental issues (water & air quality, endangered species, etc.): None
B. Biologicals
Several biologicals are available; Kodiak, YieldShield, Serenade (Bacillus subtilis), PlantShield (Trichoderma harzianum), SoilGard (Gliocladium virens). Are any used?
C. Cultural methods
Choose well drained fields that have been rotated away from cucurbits for at least one year.
D. Non-registered (Pipeline materials) pest management tools - none
BACTERIAL FRUIT BLOTCH
Bacterial Fruit Blotch is an occasional disease problem on Delmarva. Begin treatment at first bloom.
A. Chemical Control (Virginia)
1. Organo-phosphates - none
2. Carbamates - none
3. B2 carcinogens - none
4. Other chemical pesticides
Tanos
Suppression of bacterial fruit blotch
Fixed copper
Needs to be applied early and often if weather is conducive for disease development. Cannot be expected to provide control, maybe suppression.
B. Biologicals - none
C. Cultural methods
2 years between watermelon crops. Buy seed that has been tested to be free of the pathogen. Sanitation during transplant production and possibly fungicide use if the label allows.
D. Non-registered (Pipeline materials) pest management tools: none
VIRUSES
Viruses (CMV, WMV, PSRV, and ZYMV) are transmitted through aphid feeding.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens - none
4. Other chemical pesticides - none
B. Biologicals - none
C. Cultural methods
Practice strict aphid control. Plant fields as far away from existing cucurbit plantings as possible. Select disease resistant varieties if available.
D. Non-registered (Pipeline materials) pest management tools - none
AIR POLLUTION (OZONE)
Air pollution (ozone) will cause chlorosis and upper surface scorching on older leaves, which leads to defoliation.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens - none
4. Other chemical pesticides - none
B. Biologicals - none
C. Cultural methods -
Plant varieties which are less sensitive to ozone injury.
D. Non-registered (Pipeline materials) pest management tools - none
FUSARIUM WILT
Fusarium wilt is a vascular disease caused by soilborne fungus. Initial symptoms are that plants appear to wilt in the middle of the day (when transpiration is high) then recover. Leaves on one branch or stem may wilt and turn color. Finally entire plant collapses.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens - none
4. Other chemical pesticides - none
B. Biologicals
Non-pathogenic Fusaria suppress Fusarium wilt. However, these are not yet commercialized.
C. Cultural methods -
Plant in fields which have not had watermelons for 5-6 years. Adherence to a rotation schedule is important, but is not completely reliable as a control method. Use resistant varieties. Planting into a tilled hairy vetch cover crop has been tested and is effective in the mid-Atlantic. At present, growers are experimenting with adoption of practice.
D. Non-registered (Pipeline materials) pest management tools - none
ANTHRACNOSE (PRIMARY PEST)
Anthracnose is one of the most destructive foliar diseases of watermelon. All above ground parts of the plants are affected. The fruit becomes susceptible to infection at about the time of ripening. The fungus overwinters in infected plant debris in the soil and seed. Begin treatment when vines run or earlier if symptoms begin to develop.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens –
“chlorothalonil” (Bravo Weather Stick, Echo, Equus)
Mancozeb (North Carolina) Is not in our Recs book for anthracnose
4. Other chemical pesticides
Pristine--18.5 oz 38W/A, or a tank mix of chlorothalonil plus on of the following:
azoxystrobin (Amistar--3.5-5 oz 80WDG/A or
Quadris (azoxystrobin)--11- 15.4 fl 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)
B. Biologicals - none
C. Cultural methods -
Plant in fields which have not been planted with any cucurbits for 2 years. Use resistant varieties when possible.
(Virginia) Some varieties offer excellent resistance, so use them whenever possible. For disease control in susceptible varieties, it is important to combine protectant fungicide applications with seed treatments and crop rotation with noncucurbits. If infection does occur, know that the disease overwinters in infected seeds, leaf litter, and plant debris and can remain in the soil for several years. After harvest, be sure to remove all plant debris to avoid a possible re-infection.
MELCAST is a weather-based fungicide application program for watermelon anthracnose and gummy stem blight. Use of this program can reduce fungicide application 10 to 40%. This is available through UD IPM program.
D. Non-registered (Pipeline materials) pest management tools - none
DOWNY MILDEW
Downy Mildew generally does not occur until mid-August. This fungal disease is dependent on moisture and can cause heavy losses in a short time if the aggressive strain of the fungus is present and the weather is ideal for infection and spread. Begin sprays when vines run or if disease occurrence is predicted for the region.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates –
Previcur Flex (propamocarb) (North Carolina)
One of the most effective fungicides on cucumbers.
3. B1, B2 potential carcinogens –
Ridomil Gold/Bravo 76.5 WP plus Bravo WeatherStik 6 F or Terranil 6L. Not sure I would recommend using Ridomil for downy mildew. Need consensus on this. Fungus is probably resistant to this active.
IPM and/or resistance management issues: Bravo needed for resistance management as a tank mix partner as well as a rotation product.
Mancozeb (Virginia and North Carolina)
Gavel--1.5-2 lb 75 DF/A (Gavel contains mancozeb, which is
a protectant, and does not need a tank mix partner.)
IPM and/or resistance management issues: Tank mix partner for resistance management less effective than chlorothalonil
4. Other chemical pesticides –
The following are the most effective materials (tank mix these products with a protectant such as chlorothalonil--1.5-2 pt 6F/A, or OLF):
Ranman--2.1-2.75 fl. oz. 400 SC/A, or
Previcur Flex--1.2 pt 6F/A, or
Curzate--3.2 oz 60DF/A, or
Tanos (famoxadone + cymoxanil)--8 oz 50WDG/A
B. Biologicals - none
C. Cultural methods - none
D. Non-registered (Pipeline materials) pest management tools - none
ALTERNARIA LEAF BLIGHT
Alternaria leaf blight causes plants to loose leaves. This fungus also overwinters in infected plant debris in the soil and seed. Begin spraying when vines begin to run.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens –
Bravo, WeatherStik 6 F or Terranil 6L (chlorothalonil)
4. Other chemical pesticides –
Begin sprays when vines begin to run.
Alternate:
chlorothalonil--2-3 pt 6F/A, or OLF (Use low rate early in season)
With:
Pristine--12.5-18.5 oz 38W/A, or a tank mix of chlorothalonil plus one of the following every 14 days:
azoxystrobin (Quadris--11-15.4 fl oz 2.08F/A or Amistar--3.7-5 oz 80WDG/A), or
Cabrio--12-16 oz 20EG/A
B. Biologicals none
C. Cultural methods – (Virginia)
Using resistant varieties whenever possible and following a proper management routine will help reduce the risk of infection. Make sure you rotate cucurbits out with other vegetables for three or four years. After harvest, plow under or burn? any crop debris left in the field.
D. Non-registered (Pipeline materials) pest management tools - none
GUMMY STEM BLIGHT (PRIMARY PEST)
Gummy stem blight usually attacks the leaves and stems of watermelon; however, in favorable weather it can infect any plant part. This fungus also overwinters in infected plant debris in the soil and seed. Begin sprays when vines begin to run.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates –
Topsin M 70 WP (thiophanate-methyl) Our recs book does not recommend thiophanate-methyl any longer because of widespread resistance. Discuss?
3. B1, B2 potential carcinogens –
(chlorothalonil) Bravo WeatherStik 6F
Mancozeb (North Carolina)
4. Other chemical pesticides –
Alternate chlorothalonil--2-3 pt 6F/A or OLF
With:
a tank mix containing chlorothalonil plus Pristine (pyraclostrobin + boscalid) --12.5-18.5 oz 38W/A, or
azoxystrobin (Quadris) --11-15.4 fl oz 2.08F/A or Amistar--3.5-5 oz 80WDG/A), or
Cabrio--12-16 oz 20EG/A
B. Biologicals none
C. Cultural methods –
Plant in fields which have not had cucurbits for 2 years. MELCAST is a weather-based fungicide application program for watermelon anthracnose and gummy stem blight. Use of this program can reduce fungicide application 10-40%. This program is available through DE IPM program.
D. Non-registered (Pipeline materials) pest management tools - none
POWDERY MILDEW (PRIMARY PEST) (VIRGINIA) Minor pest in DE
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 in other cucurbits because sporulation is sparse and masked by leaf color. Look for chlorotic spots on upper leaf surface of young, fully expanded leaves, and then inspect the 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. Strains of the pathogen that are highly resistant to the strobilurins/FRAC group 11 fungicides (see table) were reported in the Eastern U.S. Powdery mildew generally occurs from mid-July until the end of the season. 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:
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens - chlorothalonil--2-3 pt 6F/A
4. Other chemical pesticides –
Alternate:
Nova--5 oz 40WP/A, or
Procure--4-8 oz 50WS/A,
Pristine--12.5-18.5 oz 38WG/A
Nova (myclobutanil)
Procure (trifumizole)
B. Biologicals none
C. Cultural methods - none
D. Non-registered (Pipeline materials) pest management tools - none
PHYTOPHTHORA BLIGHT
Phytophthora Blight is an emerging disease of watermelons on Delmarva; however, in Virginia This highly destructive disease can cause many different problems for a crop, including: seedling damping-off, leaf spots and fruit rot, and possibly total crop loss. Older plants, with root infections, may suddenly wilt even though they show no signs of stem or vine lesions. In fruit the symptoms begin as small, water-soaked lesions in the rind, which enlarges quickly and becomes a soft, sunken area covered with white fungal growth. This eventually will lead to fruit collapse in the field or storage. During periods of heavy rainfall, this disease can spread rapidly, quickly becoming a very serious problem. Experience in DE and Maryland has been limited to fruit rot infections.
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 for suppression only:
Forum--6.0 oz 4.18SC/A (Must be tank mixed with another fungicide that is effective against Phytophthora blight on watermelon such as fixed copper), or Gavel--1.5-2 lb 75DF/A, or Tanos--8-10 oz 50DF/A+ mancozeb, or Ranman--2.75 fl oz 400 SC/A plus an adjuvant (see label for details, do not tank mix with copper)
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens –
Gavel Mancozeb
4. Other chemical pesticides – none
B. Biologicals none
C. Cultural methods –
Plant to field which has not had a crop of cucurbits, eggplant, pepper or tomato for 3 years. Select a field with excellent drainage and use cultural practices that insure good drainage from around the base of the plant and out of the field.
D. Non-registered (Pipeline materials) pest management tools - none
CERCOSPORA LEAF SPOT (NORTH CAROLINA)
Cercospora leaf spot is a secondary pest in Delaware and Maryland. It is often present but has not been observed to cause yield loss.
A. Chemical Control
1. Organo-phosphates - none
2. Carbamates - none
3. B1, B2 potential carcinogens –
Bravo (chlorothalonil)
Mancozeb
4. Other chemical pesticides –
Quadris (azoxystrobin)
B. Biologicals none
C. Cultural methods - none
D. Non-registered (Pipeline materials) pest management tools - none
RIND NECROSIS (NORTH CAROLIINA)
A. Chemical Control
1. Organo-phosphates -
2. Carbamates -
3. B1, B2 potential carcinogens -
4. Other chemical pesticides -
B. Biologicals
C. Cultural methods -
D. Non-registered (Pipeline materials) pest management tools
Nematodes (Virginia)
The southern root-knot nematode (Meloidogyne incognita) is the most important species of nematode affecting watermelons in Maryland, Delaware and Virginia.
A. Chemical Control
1. Organo-phosphates –
2. Carbamates –
Vydate L (oxamyl) (Virginia)
Water soluble can be applied through drip irrigation and used post transplanting, effective when used as a spray when nematodes detected early
3. B1, B2 potential carcinogens –
Metam sodium (Busan, Nemasol, Vapam HL)
4. Other chemical pesticides –
Chloropicrin (Virginia)
Terr-O-Gas 67, MC-33 (methyl) (Virginia)
Telone II (dichloropropene)
B. Biologicals - none
C. Cultural methods –
(Virginia)
Exclusion and Sanitation are the best preventative measures against nematodes. Examples include obtaining nematode-free transplants and washing soil from machinery and tools before using them at different locations. Crop rotation with non-host crops to lower their population size is highly recommended in the event of nematode activity.
There are no resistant varieties.
D. Non-registered (Pipeline materials) pest management tools