Agriculture & Natural Resources

IPM – Cucurbit Scouting Guidelines

I. General Insect Scouting Guidelines

Begin scouting fields as soon as plants are transplanted or emerge from the ground. Certain insects can colonize a field very quickly and cause direct damage to emerging or newly transplanted plants. Others can transmit diseases. Twice weekly scouting may be necessary when population pressure is high and insects are developing quickly. Pay particular attention to the edges of fields, where localized “hot spots” of insect activity may be found. Before plants begin to vine, inspect each of 5 plants for general appearance and insect occurrence. After plants vine and individual plants are indistinguishable, inspect 10 leaves and 5 fruit at each of five plant areas in the field, recording insect presence and damage.

The following are the key insect/mites pests of cucurbits


SEED CORN MAGGOT

This insect is primarily a problem in early-planted fields, especially during cool, wet growing seasons. Only a few maggots per seed or plant can significantly reduce stands. Maggots overwinter as puparium in the soil with flies emerging as early as late February. Eggs are laid in freshly plowed fields as well as in greenhouse flats before transplanting into the field. Outbreaks are favored by planting into freshly plowed ground that is high in organic matter; freshly manured fields; and/or heavy crop residues (e.g. small grain covers) where spring tillage is delayed and/or surface residue is visible after spring tillage operations.

Monitoring and Decision-Making: Scouting and applying rescue treatments after the damage is observed are ineffective. Management options must be applied to high-risk fields prior to planting for direct-seeded fields or prior to laying the plastic strip for transplanting.


STRIPED AND SPOTTED CUCUMBER BEETLES

Both species of cucumber beetles are known to infest cucurbit fields. The striped cucumber beetle has a black head, yellow thorax and three black stripes along the length of its body. The spotted cucumber beetle is similar in size but has a yellowish-green body and 12 black spots on its back. Beetles overwinter in nearby hedgerows and woodlands feeding on alternate weed hosts in the early spring. As soon as melons are planted in May, beetles migrate to the field and begin feeding on young seedlings. Although most watermelon cultivars have good bacterial wilt resistance, heavy beetle populations (greater than 5 per plant) can severely affect stand establishment during the cotyledon stage. Once the first three leaves are established, plants generally compensate for damage and growth delays before harvest. In comparison, cucumbers and muskmelons are extremely susceptible to damage. Early detection and control is critical in these crops.

Monitoring and Decision-Making: Sampling should begin as soon as transplants are set in the field or at plant emergence. Sample fields twice a week, especially along field margins next to overwintering areas. During hot, windy days, look for beetles hiding in cracks in the soil surface and under the plastic mulch. Examine 5 plants in 5-10 locations throughout a field and count the number of beetles per plant. Foliar treatment will be needed if you find 2 beetles per plant and beetles are affecting stand establishment during the cotyledon stage for watermelons. Treatments may be needed sooner on fresh market cucumbers and muskmelons. Pickling cucumbers can compensate for at least a 10 percent stand loss. Beetles should be controlled before they feed extensively on cotyledons and first true leaves. Multiple applications are needed if beetles continue to re-invade fields.



MELON APHID

Melon aphids are the predominant species attacking cucurbits grown in the Mid-Atlantic region. They vary in size and color from light yellow, green to black. Infestations begin when winged forms fly to the fields in late May. In-season, only colonies of wingless forms are generally found on plants. They can be identified from other aphids by the black cornicles or “tailpipes” found on the abdomen. The cornicles are entirely black from the tip to where they attach to the abdomen. They feed mainly on the undersides of the leaves resulting in cupping of leaves, leaf distortion, plant stunting, and a reduction in the quality and quantity of fruit. In addition to feeding damage, the melon aphid is one of the chief vectors of cucumber mosaic virus. Infestations are usually higher in hot, dry summers following cool springs that reduced the efficiency of natural enemies. In addition, over fertilization with nitrogen can increase aphid populations.

Monitoring and Decision-Making: Scouting should begin as soon as plants form runners. Look for wilting and curled leaves that will be found in small-scattered spots throughout the field. Examine 5 runners in 5-10 locations throughout a field and record the percentage of runners with 5 or more aphids per leaf. The level of natural controls (e.g. lady beetles, lacewings, and parasitized aphids) should also be considered when making a treatment decision. A foliar treatment should be applied if beneficial insect populations are low and you find 20% or more of the runners infested with 5 or more aphids per leaf.



SPIDER MITES

Although spider mites can attack all cucurbits, they are a serious pest of watermelons, especially during hot, dry weather. Infested plants appear yellow and become visible from a distance. They are primarily found on the undersides of leaves making the leaves appear tan or yellow and have a “crusty appearance”. Mites feed on the plant sap and can defoliate vines in a few weeks in hot, dry weather. Defoliated plants tend to yield small, poor quality fruit.

Monitoring and Decision Making: Since mite infestations generally begin along field margins next to grassy areas, near rye windbreaks, and in the sandiest areas of fields, be sure to carefully sample these areas early in the season. Since we are limited in our insecticide control options, early detection is critical. Once populations explode in hot, dry weather, control is extremely difficult. Look for the early signs of white stippling on the crown leaves. Mites can be identified by shaking leaves onto a sheet of white paper and watching for moving specks or by using a hand lens to count the number of mites per leaf. Examine 5 crown leaves in 5-10 locations throughout a field for the presence of mites and feeding damage. A treatment should be applied when 20-30% of the crown leaves are infested with 1-2 mites per leaf. In addition, do not mow adjacent grassy areas or windbreaks that harbor mites and force mites to disperse into the field.


THRIPS AND LEAFHOPPERS

Thrips are generally a problem early in the season when plants are drought stressed. They are generally found on the undersides of leaves producing silver flecking near the large leaf veins. Examine 5 crown leaves in 5-10 locations for the presence of thrips and rate the feeding injury as light, moderate or heavy. . Although no thresholds are available, controls may be needed if the thrips population is heavy, leaf feeding is present and plants are not actively growing.

The potato leafhopper is the predominant species causing damage to cucurbits in this region. It is a migratory pest appearing in the Delmarva region in early May. Although generally a problem after bloom in cantaloupes and pumpkins, leafhoppers can damage watermelon during hot, dry weather. In combination with spider mites and thrips, economic losses can occur. Examine runners in 5-10 locations for the presence of leafhopper nymphs. A sweep net can also be used to sample for adults. Controls will be needed if “hopper burn” is detected on leaf edges and injury is expected to retard fruit maturity and yield.


II. General Disease Scouting Procedures

(a) Early Season: When plants begin to run (vine) or as bush types flower, select 5 representative sites from which to make counts. At each site inspect 2 older leaves on each of 5 plants for presence of each disease. During a wet spring particularly note presence of Septoria, scab, and angular and bacterial leaf spot, as subsequent fruit infection can occur. Record how many plants are infected. A total of 50 leaves should be inspected for the field.

(b) Later Season: After the rows close for the vining types or for bush types when plants have fruit set and fruit is enlarging, it is better to examine 10 leaves/area and 5 fruit at each location. Initial occurrence of powdery mildew will be found on summer squash so check these plantings first for the presence of mildew. Calculate and record the percent plants infected. In addition, give each field an overall disease severity rating for each disease present according to the following scheme:

1 = first occurrence of the season on leaves
2 = few leaves (1-3) of each plant infected
3 = moderate number of leaves (3-8) infected on each plant, first occurrence on stems and fruit
4 = each plant severely infected (9-all leaves), infected stems and fruit now common
5 = total vine destruction with heavy fruit infection

The presence of all diseases except Phytophthora blight, Fusarium crown and foot rot, Fusarium wilt, and Sudden wilt will be evaluated on a percent plants infected basis. Phytophthora and Fusarium crown rot and the wilt diseases will be noted when present since they can be somewhat sporadic in their occurrence on a given farm. It is also important that the different crops be scouted separately for purposes of disease evaluation; however, cross-infection is common, especially if the different cucurbits are grown in close proximity to one another.

(c) Generalized Disease Occurrence

The presence of all diseases except Phytophthora blight, Fusarium crown and foot rot, Fusarium wilt, and Sudden wilt will be evaluated on a percent plants infected basis. Phytophthora and Fusarium crown rot and the wilt diseases will be noted when present since they can be somewhat sporadic in their occurrence on a given farm. It is also important that the different crops be scouted separately for purposes of disease evaluation; however, cross-infection is common, especially if the different cucurbits are grown in close proximity to one another.

(c) Generalized Disease Occurrence

Initially at Border Rows Localized Systematic Sampling & Action Thresholds
Viruses, especially CMV introduced by aphids (other viruses may be more randomly distributed) Fusarium Crown and foot rot; Phytophthora Blight; poorly drained areas of field Angular leaf spot (a), Bacterial leaf spot (a) & Bacterial wilt (b), Scab (a, c) & Septoria leaf spot (a, c) Powdery (a) & Downy mildew (a) Alternaria leaf blight (a), Gummy stem blight (b) & Anthracnose (b),Plectosporium blight (formerly Microdochium blight) (b) & Ulocladium leaf spot (a) (Fusarium crown and foot rot, Fusarium wilt & Sudden wilt can be scouted for, but no controls are available).

Action thresholds

a = symptoms on one leaf of 25-50 examined
b = preventative when fruit begin to develop (GSB ) or if symptoms are present.
c = mean temperatures (58-64F for Septoria) (63-70F for Scab) with wet weather for spore dispersal and infection.
d = monitor insect vectors to initiate treatment.

(d) Disease Occurrence on Specific Cucurbits

The importance of each disease for a particular cucurbit crop is given, where NA = not applicable (resistant varieties exist, as noted, or plants are not susceptible); X = occurs, but not damaging levels; XX = moderate susceptibility; XXX = severe.

(1) Cucumbers

(a) Bacterial

NA, Res. Angular leaf spot – Most cucumber varieties are resistant, so if leaf spots occur on cucumber, the disease could be Ulocladium leaf spot (see below); ALS is most common still on Watermelons.

XXX Bacterial wilt — Can infect all cucurbits except watermelon and gourds – Control is based upon control of the cucumber beetle vector.

(b) Fungal

X Downy mildew – Infects all cucurbits, but pathotypes vary – Inoculum introduced into the area; requires warm and moist conditions for infection and subsequent spread

XXX Fruit Rots – most important on for pickling cucumbers ( information from Gerald Holmes, NCSU – http://www.ces.ncsu.edu/depts/pp/notes/Vegetable/vg1/vg1.htm )

 

(2) Melons

(a) Bacterial

XX Bacterial wilt — Can infect all cucurbits except watermelon and gourds – Control is based upon control of the cucumber beetle vector.

 

(b) Fungal

XX Alternaria leaf blight – Can be a problem if short rotations are used (less than 2 years). Fungus is seedborne, so could occur any season.

XX Anthracnose — Can infect all cucurbits, but pathotypes vary – A problem during warm (hot) seasons with adequate rainfall and high relative humidity.

X Fusarium crown and fruit rot – A soilborne pathogen, it can cause a crown or foot rot of many cucurbit seedlings, or a fruit rot of pumpkin and winter squash; control based on 3 year rotation.

XXX Fusarium wilt – This soilborne pathogen is specific to melon; Previous history of farm reporting loses is important; 5-7 year rotation.

XX Downy mildew – Infects all cucurbits, but pathotypes vary – Inoculum introduced into the area; requires warm and moist conditions for infection and subsequent spread

XX Gummy stem blight (black rot on fruit) — All cucurbits – Symptoms can be variable on leaves, often without characteristic fruiting bodies which appear as black dots (pycnidia or perithecia of fungus).

X Phytophthora blight — All cucurbits – Soilborne disease dependent on excess moisture (rainfall or irrigation) to spread inoculum in poorly drained areas of the field.

X Powdery mildew — All cucurbits – Occurs every season, beginning first on older leaves in crown of plant. Some melon varieties have resistance to certain races of PM.

X Scab — Most cucurbits – Important at beginning and during a cool and wet season.

X Septoria leaf spot — Important at beginning and during a cool and wet season; scout for 1-2 mm whitish spots with black pycnidia (or water-soaked spots if frequent rains have occurred). Rarely a serious problem on foliage, and no fruit infection has been noted in melons.

(c)Viral

XXX Cucumber mosaic — All cucurbits— Usually the first virus infecting melons; wide host range including lettuce, pepper, and tomato. Use of early season row covers may exclude aphids transmitting virus.

XXX Watermelon mosaic — All cucurbits— Usually the second most important virus infecting melon.

XX Papaya ringspot-type W — All cucurbits

XX Zucchini yellow mosaic – All cucurbits.


III. General Weed Scouting

Making a Weed Map: Weeds or weed species may not be evenly distributed over a field. Where localized areas of severe infestations are found or atypical conditions exist (poorly drained area, high spots, field edges), weed infestations may be recorded on a weed map. A weed map illustrates problem areas and provides information for future control decisions. When weed maps are kept over a period of years for a given field, changes in location and population can be noted and control decisions adjusted accordingly. Areas of severe infestations can be targeted for specific control practices, rather than treating a larger area needlessly or failing to control problems at all.

First, make a rough sketch of the field, including landmarks, boundaries, crop row direction, compass directions, roads, planting date, date of map preparation, and any other important details. Then the following information should be indicated on the map:

Weed species, or if this is unknown, some effort should be made to distinguish annuals from perennials, and broadleaf species from grasses and from yellow nutsedge.

Abundance of each species estimated according to the following system: 0 = None; 1 = Scattered, few weeds; 2 = Slight, 1 weed /6 row feet; 3 = Moderate, 1 weed/3 row feet; 4 = Severe, > 1 weed/3 row feet

Distribution of weeds in the field is important and can be rated as follows: SPOTTY – found in a few places around the field; LOCAL – found in a small portion of the field; GENERAL – found throughout the field

Weed size – The following size ratings can be used: WHITE SPROUTS – seeds are just germinating or emerging; TINY – weeds show only cotyledons or first true leaf; SMALL – weeds less than 1″ tall or less than the diameter of a quarter; LARGE – weeds more than 1″ tall or more than the diameter of a quarter.

Throughout the season, at least two weed maps should be prepared. Timing should be as follows:

1. Early – soon after planting: Purpose: to evaluate the success of the current season program.

2. After Harvest. Purpose: to evaluate next season’s weed control needs.

 

University of Delaware IPM Program


Joanne Whalen – Extension IPM Specialist

Bill Cissel – Extension IPM Agent