Cucurbits at Risk for Downy and Powdery Mildew

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Note: Please read labels carefully as some of the fungicides mentioned in this article are not labelled on all cucurbits.

Powdery Mildew on Cucurbits
Powdery mildew on cucurbits is now beginning to progress. The powdery mildew pathogen is windborne and, unlike many other pathogens, can efficiently infect during dry periods such as we’re experiencing. Effectively managing powdery mildew requires fungicides. However, because the pathogen is prone to resistance, fungicide resistance within the pathogen population (Podosphaera xanthii) must be considered. Bioassays to evaluate the presence of resistance throughout the region have been conducted in the past year. Quintec (FRAC 13), Luna products (FRAC 7), and Vivando (FRAC U8) were all highly effective throughout the Mid-Atlantic and Northeast. Torino (FRAC U6) was effective in some locations but moderately effective in others including in my Maryland trials. Remember that the FRAC group 11 and 1 fungicides, which include strobilurin fungicides like Flint, and Topsin M, are ineffective. FRAC group 3 and 7 fungicides, which include Myclobutanil (Nova), boscalid (one of the active ingredients in Pristine), Fontelis and Folicur are in an intermediate group. We do know that resistance to FRAC groups 3 and 7 can be found in our pathogen populations, but if these products are used judiciously and in rotation with other effective products, they can be useful. Always tank mix fungicides with broad spectrum materials such as chlorothalonil, and alternate with a fungicide that has a different mode-of-action (FRAC group). A good strategy is to use moderately resistant cultivars and then alternate fungicides in FRAC groups where resistance has not been detected with fungicides in FRAC groups 3 or 7.

Pumpkin leaf that has both white powdery lesions, and brown downy mildew lesions. It is important to be sure that you are spraying for the correct disease because the most effective fungicides for each disease are different.

Downy Mildew on Cucumber
Downy mildew on cucumber has now been confirmed in Salem County, NJ. All cucumbers should be protected with targeted fungicides. Other cucurbits should be scouted aggressively for the presence of downy mildew.

Sunburn in Fruiting Vegetables and Fruit Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recent weather has produced conditions where there is high potential for sunburn in fruits and fruiting vegetables. Growers may need to consider ways to protect against sunburn. Sunburn is most prevalent on days with high temperatures, clear skies and high light radiation. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruits. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis. Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn. Damaged tissue is often white in color.

Storms that cause canopies in vine crops to be more open will expose fruits to a high risk of both sunburn necrosis and photooxidative sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages.

For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. These materials are kaolin clay based, calcium carbonate (lime) based, or talc based and leave a white particle film on the fruit (such as Surround, Screen Duo, Purshade and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

While particle films have gained use in tree fruits, their usefulness in vegetables is still unclear. Research in a number of states has shown reduced fruit disorders such as sunburn in peppers and white tissue in tomatoes when applied over those crops. Watermelon growers have used clay and lime based products for many years to reduce sunburn in that crop in southern states.

There are some drawbacks to the use of particle films. If used for sunburn protection on fruits, there is added cost to wash or brush the material off at harvest. Where overhead irrigation is used, or during rainy weather, the material can be partially washed off of plants, reducing effectiveness and requiring additional applications. Produce buyers can also have standards relating to the use or particle films and may not accept products with visible residues.

Bacterial Wilt Problems in Cucurbits this Year

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In a sentinel plot of cucurbits (cantaloupe, cucumber, watermelon, pumpkin, etc.) near Cambridge, MD on the Eastern Shore and in a few other cantaloupe and cucumber fields are some of the worst cucurbit bacterial wilt (Erwinia tracheiphila) infections I have seen in the past 5-7 years. Most of the infected plants are still small and were fed on by cucumber beetles 2-2.5 weeks ago. The first sign of bacterial wilt infection is when leaves near the base of the plant wilt and turn a brownish-gray/green and then dry up (Fig. 1). Then other leaves on the vine with those first dying leaves will begin to flag and wilt in the mid-afternoon (Fig. 2). In a few more days the entire vine will wilt. Sometimes that maybe the only problem but often another vine will start to wilt and then another until the plant is dead. After bacteria enter the plant it takes anywhere from 2-4 weeks for an infected plant to wilt and die.

So far this year about 18% of the cantaloupe and cucumber plants have begun to wilt. Normally I see 3-6% of plants wilt down at this plant size. I don’t think it was an unusually high striped cucumber beetle population (these beetles act as vectors for E. tracheiphila) although a few areas had very high numbers (15-20/plant). It appears that a greater percentage of beetles were carrying the bacteria than what we would normally encounter. In Figure 3 this level of feeding damage would usually lead to about 35-40% of the plants going down to bacterial wilt, this year it is 65-75% of plants like this going down to wilt.

Under this sort of pressure applying neonics to plants while they were in the tray or that were drenched at planting (which is usually sufficient) often will not be enough to hold back beetle transmission of the bacteria 7-10 days after treatment. Foliar sprays with pyrethroids would be needed. But how do you know when more beetles are going to act as vectors—you don’t. And that is the problem, next year do you over treat because of one outlier season or continue with what you have been doing? My guess is that this is a onetime blip that so many more beetles were infective than normal. If your cantaloupe or cucumber plants look good and do not have any more than the usual amount of bacterial wilt you can consider your striped cucumber beetle management to be good.

Figure 1. The base-leaves of an infected vine begin to wilt and then dry up and die

Figure 2. After 7-10 days leaves on the infected vine become flaccid

Figure 3. Heavy beetle feeding on cantaloupe plant

Vegetable Recovery from Temporary Flooding

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

On June 9, at our Georgetown research station, we received 4.2 inches of rainfall in a one-hour period. Many of our vegetable research plots were temporarily flooded and most of our plasticulture research area had water that topped the beds.

If this flooding would have lasted for more than 24 hours, most of our crops would have been lost. However, there was enough drainage early on so that the majority the area has come through, albeit with some complications. The following are some pictures that show the recovery and losses:

Pepper plants with leaf drop. Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops. The bed area around this plant was saturated for two weeks. For the plant to recover, the bed must dry out to allow the plant to regenerate roots. The pepper plant has additional sets of buds to allow for new leaves to form but the recovery, if it happens, will put the plant several weeks behind other non-affected plants. Another concern is potential for root rots. In severe cases, cutting away the plastic can save a crop by allowing for better drying out and aeration.

Watermelon plant in saturated bed conditions after flooding showing little growth. Oxygen starvation to vegetable roots will cause roots to cease to function resulting in plant stunting, or collapse, with limited recovery potential. For this plant to recover the bed must dry out and aerate enough for the plant to fully regenerate roots.

Flooded pickling cucumber planting. Soils in in this area stayed saturated for 3 days while seeds were germinating. Lack of oxygen caused seeds to deteriorate and plants did no emerger in much of the area.

Tomatoes fully recovered from temporary flooding. In this case the crop did not drop leaves and was able to regenerate new roots quickly.

Our watermelon variety trial stopped growing and aborted fruits after the flooding but has now fully recovered and plants are starting to fill in. The beds were allowed to dry out for 9 days after flooding and then the crop was fertigated with nitrogen and sulfur. New fruits are now being set but the trial will have few early watermelons.

Fruit Set and Fruit Carry in Cucurbit Vine Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Watermelons and cantaloupes are setting fruit, pickle harvest is underway, summer squash is in full production and pumpkin planting is finishing up. With the many rainy days as well as high winds and storm damage this season, fruit loads in some early planted vine crops has been affected and higher numbers of quality defects are evident.

Lack of fruit set can result from a lack of pollination due to reduced bee activity, reduced pollen viability, or reduced pollen germination in high heat. As an example, a female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee. Research has shown that over 20 visits may be required to achieve full set and full size in some cucurbits.

This year (2018), early fruit set in watermelon and cantaloupe may be off due to poor weather during early flowering. Bees flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Bees also do not fly much below 55°F, so on cold mornings, as we often had in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.

In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather vine crops closest to the hives will have the best set and furthest from the hives will have the worst.

This year another problem is that some watermelon fields have lost significant numbers of pollenizer plants due to poor weather conditions during or after planting. This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.

Another common question from growers and crop consultants is how many fruit should a plant carry and what will affect fruit “carry” in vine crops.

For watermelons, a healthy, vigorous plant may set 3-7 fruits initially. However, for mid-size and larger watermelons, the plant will only carry 2-4 fruit at any time. Smaller fruited varieties will more fruits per plant but essentially the same total pounds as larger types. This is the carrying capacity of the plant and is directly related to the quantity of photosynthates being produced by the plant, mostly in the leaves. Any additional fruits, even if initially set, will be aborted. Once the first fruit ripens and is harvested, additional sets can be carried. To carry the maximum amount of fruit, it is necessary to maintain high plant vigor and good foliage health. This requires paying close attention to irrigation and fertility programs; having excellent disease, insect, and mite control; and having good pollinator activity during pollination and fruit set. In watermelons, if average fruit carry is less than 2 per plant, this is a sign that the plants have reduced vigor and are under stress. Repeated fruit set depends on maintaining vine health through the season.

Another factor to consider is where fruit set is occurring. Crown sets are desired in watermelons, especially in early plantings. Crown sets are those that occur on nodes closest to the base of the plant, within the first 8 nodes. Having good crown sets requires that plants have good early growth so that adequate leaf area is produced that can support early set fruit as well as proper pollination (sufficient bees). Lack of crown set is a sign of poor early growth, early plant stress, or of problems with pollination.

Growers with early-planted watermelons this year (those planted the last week in April or first 2 weeks in May) are likely to see reduced crown set and may see increased numbers of seedless melons with defects such as distinct lobes (noticeably triangular) or hollow heart and standard seeded pollenizers with pinched ends. These are signs that pollination was lacking during early fruit set. This can occur when there is a lack of pollen – pollenizers have not produced enough male flowers or are delayed in producing male flowers. In some years, fields have had losses of pollenizers, due to the poor weather during transplanting, requiring replanting. This may reduce pollen for the first set female flowers in triploids, reducing crown sets. Reduction in bee activity during the stormy weather this June may also reduce early sets. Early plant stress such as the wind damage, flooding, and hail damage we have seen this year can also cause abortion of flowers leading to reduced crown set.

With pumpkins, harvest is limited to those fruits set initially, because pumpkins are not repeat harvested as are watermelons. Medium sized Jack-o-lantern types will carry 1-2 fruits, larger types closer to 1. All others will be aborted. Smaller types will carry more depending upon their size in pounds (for example a variety with 5 lb average will carry 4-7 fruits). Maximum carrying capacity in pumpkins is largely affected by variety (varieties with some heat tolerance will carry more fruits in our climate) and foliage health. Excess nitrogen fertilization will often delay fruit set in pumpkins.

In gynoecious cucumbers grown for once over pickle harvesting, there will be two fruits set on adjacent nodes that are ready for harvest at any one time. These will be set on nodes 2-6 commonly. The pollinizers that make up a small percentage of the population will set pickles every fifth node generally and therefore only one fruit will be ready for harvest. Yield reductions in gynoecious pickling cucumbers occur when there is a loss of set so that fruits are not on adjacent nodes. Parthenocarpic pickle varieties that set fruit without pollination will commonly have 3-5 pickles on adjacent nodes ready for harvest at any one time. This allows them to be planted at much lower densities.

Downy Mildew Resistant Cucumbers

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With downy mildew being found in pickling cucumbers on Delmarva, resistant varieties should be considered.

We now have several slicing and pickling cucumbers with some degree of resistance to the current aggressive cucumber downy mildew strain. According to Clemson University new research has identified two strains of cucurbit downy mildew: A1 attacks cucumber and cantaloupes (the most aggressive strain), while A2 attacks watermelon, pumpkins, and squashes.

In gynoecious slicing cucumbers, Bristol, SV3462CS and SV4719CS and associated pollenizers have intermediate resistance to cucumber downy mildew. In gynoecious pickles, Citadel, Peacemaker, and SVCN6404 with associated pollenizers have intermediate resistance. Next generation downy mildew resistant varieties are being tested this year on Delmarva.

These varieties are not immune to downy mildew, but they show less disease under the same pressure and will out yield susceptible varieties. Under disease pressure they perform best with a fungicide program using downy mildew materials. Research is being conducted to see if there is the potential to reduce fungicide applications.

Shown in the table below is the yield of pickling cucumber varieties with and without intermediate resistance to cucumber downy mildew with one fungicide application for downy mildew under moderate disease pressure. Note that the resistant varieties significantly outperformed the susceptible variety (Georgetown, DE, 2017).

Variety Yield Bu/a  
SVCN6404 + SV2987CL pollenizer (Resistant) 329 a
Citadel + SV2987CL pollenizer (Resistant) 276 b
Vlaspik + Sire pollenizer (Susceptible) 214 c
p 0.001

Downy mildew pressure with susceptible and resistant varieties planted side by side in a trial located on the Eastern Shore of Maryland in 2018.

Watch for Aphids in Melons

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

Several reports lately of very deformed watermelon plants, but also of cucumber and cantaloupe plants. These distortions (Fig. 1) are being caused by melon aphids Aphis gossypii Glover in most cases. Melon aphids are small and range in color from a light/dark green mottle (Fig. 2), which is most common to whitish, yellow (seen during hot, dry weather), pale green, and dark green almost black forms. The legs are pale with just the tips of some parts black. The cornicles also are black. One trait of melon aphids that make them particularly difficult to manage is that unlike other aphids, their populations do not fade with higher summer temperatures. Immatures look like adults, only smaller.

Female melon aphids give live birth to clones of themselves during spring and summer and their populations can increase very rapidly especially when hidden on the underside of foliage. One of the things to look for to see if you have an actively growing aphid population is white cast skins of the aphids. Aphids must shed their skins to grow so lots of skins show that the aphids are actively growing (Fig. 2, orange arrows). The faster they grow the faster they become adults and can begin to reproduce. The reproductive period lasts about two weeks with a female producing 65-85 offspring in that time. The ideal temperature for reproduction is around 70-80°F., which are the temperatures we are experiencing now. There is evidence that there are host races, i.e., melon aphids reared on cotton can be transferred successfully to okra but not to cucurbits. This inability to transfer from one host to another has been shown for other crop combinations.

Melon aphids feed on the underside of leaves and can be a major problem on young plants when they feed near the tips of vines, sucking sap and nutrients from the plant. Their feeding causes a great deal of distortion and leaf curling, hindering the photosynthetic capacity of the plant (Fig. 1). The foliage may become chlorotic and die prematurely. They also secrete a great deal of honeydew which allows the growth of sooty mold and further reduces the photosynthetic ability of the infested plant. One of the other major problems with melon aphids (as with other aphid species) is that they are good at transmitting potyviruses such as cucumber mosaic virus, watermelon mosaic viruses, and zucchini yellow mosaic virus. It must be noted that these viruses are transmitted despite insecticide applications, which include oil sprays. This is mostly because the aphids can transmit these nonpersistent viruses within 15 seconds of reaching the plant.

Figure 1. Watermelon plant with heavy melon aphid population

Management
No thresholds have been established for melon aphid in cucurbits. Reflective mulches laid before planting can repel aphids from plants reducing or delaying virus transmission, until vine growth covers-up the plastic. In smaller fields, row covers can be used. Biological control can have a significant impact on aphid populations and is our first line a defense. Therefore, weekly sprays of insecticides should not be used in watermelon unless really needed. Because cantaloupe and cucumber are very susceptible to bacterial wilt disease, which is vectored by striped cucumber beetles several insecticide sprays may be necessary. However, resistance by melon aphids to organophosphates and pyrethroid insecticides is common. Using neonicotinoids for beetle control will help control aphids, but the neonics should not be sprayed exclusively and pyrethroids or other insecticide classes should be used intermittently for beetle control.

Figure 2. Melon aphids on underside of leaf

While many of the above suggestions are all good to prevent aphid problems what do you do once you have them? Organically there are not many good aphid control tactics to use once they show up. Applications of rosemary oil or insecticidal soaps or horticultural oils are options. These will have to be applied several times with thorough coverage of the foliage being critical for control of the pest. Rosemary oil will disrupt beneficial populations less so than soaps or oils. There are several synthetic controls that will work if thorough coverage is obtained. These chemical controls include: methomyl, dimethoate, acetamiprid, clothianidin, thiamethoxam, pymetrozine, flonicamid and combination products that include one of these. Be sure to read the label before applying any chemicals. It should be noted that a plant damaged as severely as the one in Figure 1 will not recover to produce a crop.

Alert! Cucurbit Downy Mildew in Dorchester Co.

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Downy mildew on processing cucumber was found on Thursday, June 13, 2018 in Dorchester County, MD. This is one of the earliest occurrences of downy mildew in Maryland. Protect cucumber crops with products that are specific for downy mildew. Among the specific fungicides for this disease are Orondis Ultra or Ranman, which should be mixed with a protectant fungicide. Many additional fungicides are registered for downy mildew and are listed in the Commercial Recommendation Guide. Remember to rotate products in different FRAC groups and to apply preventative fungicides, which are more effective than “rescue treatments”.

At this time, only cucumber (both processing and fresh-market) should be affected. The strains that infect other cucurbits have not been observed in our region.

Importance of Leaf Cover in Fruiting Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

July is the month that we see the highest temperatures and often have cloud free, high light intensity days and long day lengths. Under these conditions, good leaf cover is essential for producing high quality fruits. Lack of leaf cover will expose fruits to high levels of radiation and cause excessive heating of the fruit surface. This can lead to a variety of disorders including sunburn, sunscald, fruit yellowing, fruit cracking, and shriveled fruit.

Lack of leaf cover often occurs due to storm damage where high winds or hail damage leaves. After damaging storms, attempts should be made to promote new leaf cover as quickly as possible by sidedressing or fertigating with nitrogen fertilizer and by irrigating.

A second, temporary loss of leaf cover occurs during hot periods when plants are allowed to wilt. Just a few hours without cover under high heat and light can cause severe damage to fruits. This is most severe in dark colored fruit such as peppers and cucumbers. Irrigation management is critical to limit fruit damage due to wilting.

Lack of leaf cover can also be due to lack of plant vigor and poor plant growth which may have a variety of causes such as underfertilization, deficiencies, water stress, wet soil, compacted soil, hot soil conditions or other soil, water, or fertility related issues. Finding the root cause will be critical to address and correct these growth limiting factors and improve leaf cover.

Diseases that reduce leaf production, attack leaves, or cause wilting can reduce leaf cover and lead to fruit disorders. Leaf feeding insects can also contribute to leaf area losses. Protecting plants against expected diseases and insects along with scouting for signs of infections or infestations is critical to maintain canopies. Air pollution damage can also cause losses of leaf cover in sensitive crops and varieties.

Staking and pruning practices are also important to manage leaf cover. Excessive pruning of tomatoes can expose fruits to excess radiation leading to fruit damage. Single or double stem training systems, as are often used in greenhouses and high tunnels, are at most risk. Staking peppers has been shown to reduce fruit damage by maintaining leaf cover over developing pepper fruit.

One common problem in high radiation exposure conditions and lack of leaf cover is sunburn. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruits. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis (i.e. 115 to 120°F in apples). Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn. Damaged tissue is often white in color.


Sunburn necrosis on pepper.


Photooxidative sunburn on pepper.


Fruit yellowing in cucumber due to loss of chlorophyll with exposure due to inadequate leaf cover.

Fruit and Fruiting Disorders in Summer Squash and Cucumbers

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

A number of fruit and fruiting disorders have been observed in summer squash and cucumbers over the last two weeks including lack of fruit set, bottlenecking, pinched blossom ends, crooks, nubs, hollow centers or cavities, fruit zippering and scarring.

Lack of fruit set can result from a lack of pollination due to reduced bee activity, reduced pollen viability, or reduced pollen germination in high heat. Water stress will compound this problem. When day temperatures are in the 90s and night temperatures are in the high 70s, plants will commonly abort fruits or produce misshapen fruits. To reduce losses due to heat, apply irrigation so that plants are never under water stress.

Growers should note that some squash (mostly zucchini) varieties will still set fruit without pollination. Steve Reiners at Cornell did a trial in 2013 with 21 varieties of summer squash to determine which were capable of setting fruit without pollination. Female flowers were bagged prior to opening to exclude pollinating insects. After 1 week, bags were removed and fruit rated as to whether it was marketable or not. The results can be found at this web site: http://www.hort.cornell.edu/expo/proceedings/2014/Vine%20crops/Seedless%20squash%20Reiners.pdf. For example Golden Glory and Dunja Zucchini both were able to set a high percentage of fruit without pollination. Selecting varieties with this ability can reduce losses due to poor pollination.

Parthenocarpic varieties of cucumbers and zucchini that set fruit without pollination are also available and can be less susceptible to environmental extremes or conditions that limit bee activity in monoecious or gynoecious varieties. We currently are evaluation 17 parthenocarpic pickle varieties for adaptation to our region.

Lack of fruit set can also be due to harvest management. When summer squash or cucumbers are allowed to progress to an overly mature stage, plants will “shut down” and not reflower for a period of time. To manage this problem, frequent picking (every 2-3 days) is necessary.

Misshapen fruits commonly are found in high numbers with high temperatures and water stress in the summer or low night temperatures in the fall. This includes bottle necking, pinched blossom ends, crooked fruits or fruits with “narrow waists”. These defects are most commonly due to effects on pollination. Other stresses such as herbicide injury, root pruning in cultivation, or wind damage can increase the number of misshapen and unmarketable fruit. Potassium deficiency can also cause pinching at the stem end.

Hollowness or open cavities in cucumber and summer squash fruit can be caused by inadequate pollination and reduced seed set. Boron deficiency or the combination of boron and calcium deficiency can also result in increased hollowness.

Progression from marketable to unmarketable pickle fruits that are crooked, waist pinched, tip pinched or tip pinched with crook.

Small cavities in cucumber fruit. In a more severe form hollowness and cavities can render the fruits unmarketable or reduce processing (pickling) quality.