Posts Tagged ‘cantaloupe’

Yellow Cantaloupe Leaves

Friday, August 10th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Each year we see problems with cantaloupe leaves turning yellow. There are several potential causes. If the yellowing is on leaf edges it most commonly is due to salt effects and fungicides, see the article by Jerry Brust two weeks ago for more details Copper fungicides are often the culprit in this leaf yellowing, causing a phytotoxic reaction. Foliar fertilizer applications can often worsen the yellowing by increasing salt levels on the leaves.

Each year there are some fields of cantaloupes that are affected by manganese toxicities. This occurs when bed pH drops below 5.4 which affects soil chemistry so that plant available manganese increases greatly and plants take up quantities that become toxic. As a micronutrient, Manganese is needed in only small amounts and the sufficiency range is between 20-100 ppm. Magnesium deficiencies also can occur at low pH and older leaves will show interveinal chlorosis. These symptoms can be confused with mite damage so check for mites in the diagnostic process.

Air pollution is another cause of yellowing of cantaloupe leaves. This yellowing is usually confined to older crown leaves.

Cantaloupe leaves showing signs of salt injury, copper fungicide phytotoxicity and managanese toxicity. Bed pH was 5.4.

Striped Cucumber Beetle Populations Still Very High

Friday, July 27th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland; and Karen Rane, Extension Specialist Entomology, University of Maryland

We have seen very high populations of striped cucumber beetles on squash, pumpkins, cantaloupe, watermelon and other cucurbits over the last few weeks. These populations at times have reached over 20-30 beetles per plant. If a spray was missed or plants were not thoroughly covered with an insecticide application the beetles would soon consume that unprotected area very rapidly (Fig. 1). This area is often times the base of the plant. This is especially true if the cucurbits are sprayed with an air-blast sprayer. While air-blast sprayers do a good job of covering leaves with material, they often do not do a great job of covering the base of a plant and heavy feeding can occur (Fig. 2). The feeding can lead to plants being girdled by beetles or can lead to bacterial wilt infection—even though the leaves of the plant show almost no feeding. This feeding by the beetles also opens the base of the stem to infection from soil organisms and greater rates of Fusarium and bacterial soft rots are possible. When beetle populations are this high the base of the plant, even more so than the foliage, needs to be protected from heavy feeding.

Figure 1. Two squash leaves fed upon heavily by striped cucumber beetles because of the lack of good spray coverage.

Figure 2. Base of pumpkin plants damaged by striped cucumber beetles due to poor spray coverage

Marginal Chlorosis on Cantaloupe Leaves

Friday, July 27th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

I have been seeing and receiving a few samples of yellowing (chlorosis) around the margins of cantaloupe leaves (Fig. 1). There was no chlorosis between the veins and no marginal necrosis (these two symptoms would indicate molybdenum deficiency). The marginal chlorosis would indicate salt burn, which is a noninfectious problem that mostly affects cantaloupes. This yellowing at the leaf margin is likely the result of guttation, which is how plants exude water at the margin of the leaf. Water droplets from the plant accumulate at the edge of a leaf in the early morning. This water often contains organic and inorganic compounds, and mineral nutrients, especially potassium. As the water evaporates these compounds are left behind and concentrat­ed at the leaf margin. Over time the high concentration of compounds shows up as a marginal yellowing called ‘salt burn’. These deposits also can have a toxic effect on the gas exchange pores (the hydathodes) located at the leaf edges. Salt accumulation often is associated with foliar applications of nutrient solutions or pesticides during very hot, dry weather. Frequent copper applications when it is hot and dry also can result in distinct bands of yellow tissue around leaf margins (Fig. 2). Soil applied urea or ammonium nitrate fertilizers may contribute to the problem as well. In most cases there is no yield loss as a result of these symptoms and no control measures are needed; however there could be yield losses if salt burn is severe and widespread in the field.

Figure 1. Marginal chlorosis on cantaloupe leaves (arrows) caused by salt burn

Figure 2. Marginal chlorosis most likely caused by copper sprays and salt burn

Pollination Disorders in Cucurbits

Thursday, July 12th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Watermelon harvest is underway on Delmarva; cantaloupe harvest started early this year, squash and cucumbers have been producing for over a month; and pumpkins and winter squash are setting fruit in earlier plantings. Each year, we see pollination problems with vine crop fruits, especially when weather conditions are unfavorable.

Signs of incomplete pollination in cucurbits include bottlenecked fruit or fruit with a pinched end, crooked or lopsided fruit, fruit small in size or nub-like; and fruits with prominent lobes or that are triangular in shape. Causes of incomplete pollination may be inadequate pollen transfer by pollinating insects; inadequate pollen sources (pollenizers); or hot, dry weather that reduces pollen viability or that desiccates flower parts during pollination. Research has shown that a minimum of 1,000 grains of pollen are required to be distributed over the three lobes of the stigma of the female flower of a watermelon to produce a uniformly shaped fruit.

Hollow cavities in fruit and vacant seed cavities are related to lack of seed formation, again traced back to poor pollination. Fruit tissue separation, such as hollow heart in watermelon, may also be due to inadequate pollination and may be worsened by rapid fluctuation in environmental conditions affecting fruit development.

High Evapotranspirational Demand in Vegetable Crops

Friday, July 6th, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

Vine crop growers have commented that they are having a hard time keeping enough moisture in their beds. Certainly, irrigation system concerns need to be accounted for (emitter volume, emitter spacing, length of run, etc.). However, high evapotranspirational demand (water taken up by the plant and evaporated from the soil) may make providing enough irrigation water a challenge.

This is particularly the case when there are high temperatures and clear skies during June and early July when day lengths are long and when plant water uptake is peaking. In 2012 during this period we have had consistently higher solar radiation and higher evapotranspirational demand compared to 2011.

Leaf Aging in Cucurbits

Friday, June 22nd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

We are starting to see the oldest leaves (crown leaves) in watermelons, cantaloupes, squash, cucumbers, and pumpkins with large areas that are discolored (white, tan, or bronze). These leaves will be brittle to the touch and may start to tear or shred with high winds and storms. This condition is common in cucurbit crops and can be due to a number of leaf aging factors including mineral nutrient scavenging (export of mobile nutrients from oldest leaves to newer leaves), ozone air pollution damage, chemical phytotoxicity, repeated stress cycles, and wind injury. Leaf cells that die will leak their contents, releasing enzymes and oxidizing chemicals affecting nearby cells thus accelerating the “aging” process. This results in large patches of dead leaf cells that then dry, making the leaf feel brittle. If leaf veins are damaged, water and food transport will be compromised, accelerating leaf decline. This leaf aging is not to be confused with damage from mite feeding which is also concentrated on oldest leaves.

Cucurbit Downy Mildew Alert!

Friday, June 15th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Cucurbit downy mildew was confirmed in Dorchester and Caroline counties in Maryland on June 12. Symptoms first appeared on June 8. This downy mildew occurrence is one month earlier than any occurrence in recent years. Growers should scout aggressively for this disease on cucumber and other cucurbits. This disease is favored by cool, humid weather including cool dewy nights. Weather during June 12-14 is forecast to be conducive to further spread. Tank-mix Ranman or Previcur Flex with a protectant fungicide and alternate sprays with a material with a different mode of action. Be careful not to rely on one fungicide class. Use excellent resistance management practices to avoid allowing the pathogen to develop resistance and to improve the efficacy of your fungicide management program. Presidio, which was commonly used in previous years, was not as effective as expected in 2011 University trials.

Downy mildew on the lower surface of a cucumber leaf. Notice the angular, water soaked lesions on this newly infected leaf. (Image courtesy of Bugwood and Gerald Holmes)

Consult the Commercial Vegetable Production Recommendations for further information on resistance management and available fungicides (in Maryland, Extension Bulletin 236 and in Delaware, Extension Bulletin 137). Because downy mildew has only been found on cucumber, targeted sprays on other cucurbits crops such as pumpkin, squash, watermelon, etc. are not necessary, at this time. Instead scout these crops aggressively and continue to apply a broad-spectrum spray program.

Cucurbit Downy Mildew Fungicide Decisions

Friday, May 11th, 2012

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Downy mildew on cucurbits has been a problem on Delmarva beginning in early July for the last few years. Good fungicides for management are available. However, last year in my trials, one of these fungicides, Presidio, was not as effective as expected. Looking ahead to your spray program, be careful not to rely on one fungicide class. It is difficult to know which fungicides will be effective here, because our population does not overwinter and is reintroduced from the South each year. Therefore use excellent resistance management practices to avoid allowing the pathogen to develop resistance and to improve the efficacy of your fungicide management program.

Fungicide Resistance management guidelines by crop are available online and hard copies are available in Delaware at the county Extension offices.

Inspect Watermelon and Cantaloupe Transplants, New Bacterial Fruit Blotch Factsheet

Thursday, May 3rd, 2012

Gordon Johnson, Extension Vegetable & Fruit Specialist;

As watermelon and cantaloupe transplanting continues on Delmarva, growers are reminded to inspect plants before they are transplanted into the field for signs of disease including Bacterial Fruit Blotch, Gummy Stem Blight, and Angular Leaf Spot.

Kate Everts and Gordon Johnson have put together a new factsheet on Bacterial Fruit Blotch, which will be of interest to watermelon growers. It is available online here:

Seedcorn Maggot and Cabbage Maggot Damage Possible in the Next Few Weeks

Thursday, April 26th, 2012

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

The unusually warm and dry spring we have had up to now has led many growers to transplant some of their melon and other vegetable crops early. The cool wet weather we have had in the last few days will make some of these fields vulnerable to seed corn maggots Delia platura (SCM) or less commonly found in cucurbit fields, cabbage maggots, Delia radicum (CM). Both species overwinter in the soil as a maggot inside a brown case. In March and April small, grayish-brown flies emerge. Adult flies are most active from 10 a.m. to 2 p.m. and are inactive at night, in strong winds and when temperatures are below 50o F or above 80o F. Female cabbage maggot flies seek out and lay eggs on the lower portions of stems of young host seedlings or in nearby cracks in the soil. Within a few days the eggs hatch and the tiny maggots burrow down to the roots and begin feeding. SCM eggs are oviposited in soils with decaying plant material or manure. The adults are also attracted to the organic media around the roots of transplants and germinating seeds. That is why fields that have been fumigated can still have problems with SCM. Maggots will move into small stems and move up the plant causing a swelling of the stem just above ground level, while also causing root collapse and decay. If these stems are split you will usually find the white cylindrical larvae (Photos 1, 2 and 3).


Photos 1, 2 and 3. Swollen stem of cucurbit plant with collapsed rotting roots. When stem is cut open the white maggots often can be found.

The adult flies are often found dead, stuck to vegetation during periods of warm wet weather (like we had in early April). These flies have been infected by a fungus, Entomophthora sp. These infected flies usually will be found at the top of a tall object in the field such as a grass seed head or a wire field-flag (Photo 4). Just before the fungus kills them they cement their body via their mouthparts to the tall object and die. If you look closely you’ll see the body is filled with the white fungus that has ruptured between the segments (Photo 5). Being on a tall object allows the spores of the fungus to move longer distances and infect more flies than if the fly had died on the ground. Even though we have had a dry spring, I still have seen many fungus infected dead flies this year. Unfortunately, the infection rate is not enough to reduce the SCM population and stop infestations.

Soil temperatures two inches deep in the planting hole that are at or above 70o F reduce SCM egg laying and larval survival. If soil temperatures are above 70o F at planting but fall below this level for several days in a row (which they have just done), SCM adults will begin to oviposit eggs at the base of transplants. When wilted transplants are inspected in the field, maggots are often not found (they have already pupated), but their tell-tale damage can be seen as a hollowed out stem or root held together by a few strands of plant material. The use of treated seed or in-row banding of an insecticide gives some control of SCM, however, replacing dead transplants is the only solution after SCMs kill a plant. Once seed corn maggot damage is noticed, it is too late to apply control procedures. Thus, economic thresholds are not useful and all management options are preventative.

Photo 4. Two SCM flies killed by a fungus stuck to a wire field-flag via their mouthparts

Photo 5. Adult SCM killed by a fungus – white strands coming out of abdomen