Spots on cucumber leaves

Cucumber (Cucumis sativus)-Angular Leaf Spot

Cause Pseudomonas syringae pv. lachrymans, a bacterium that overwinters in diseased plant material and on seed. It is spread by rain, sprinkler irrigation, and on workers’ hands and clothing. In recent years, using tolerant cultivars has reduced disease incidence.

Symptoms Leaves, stems, and fruit may be affected. The leaf spot is irregularly shaped with a water-soaked appearance; bounded by the leaf veins the shapes of spots are angular. Bacteria may ooze from the spots in droplets, which dry to a white residue. The water-soaked area later turns gray and dies. Often, dead tissue is torn away from the healthy portion, leaving large, irregular holes. Water-soaked spots on fruit are smaller than on leaves, and they are circular. Lesions usually are superficial, but the injury may permit entry of soft-rot organisms. Affected tissue becomes white and may crack open.

Cultural control The following control program is a combination of those used in other areas.

  • Use clean, bacteria-free seed.
  • Practice a 2-year rotation out of cucurbits.
  • Plant resistant cultivars. Pickling cultivars Regal, Royal, Pioneer, Express, Calypso, Cross Country, and Frontier have shown tolerance under Washington and Oregon conditions. Slicing cultivars Victory, Bel Aire, Raider, Encore, Poinsett 76, Slice Nice, Dasher II, Turbo, Quest, and Sprint-N are resistant.
  • Stay out of wet, infected fields.
  • In the Hermiston, OR area, do not irrigate with water draining from another cucumber field.

Chemical control

  • Actigard 50WG at 0.5 to 1 oz/A on 7-day intervals. May be applied the day of harvest. 12-hr reentry.
  • Applying fixed-copper products has reduced disease spread but reportedly has caused some stunting and leaf chlorosis in Oregon.
    • Badge SC at 0.5 to 2.5 pints/A on 5- to 7-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
    • Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. Poor control as a stand-alone product. 4-hr reentry. O
    • Cuprofix Ultra 40D at 1.25 to 2 lb/A on 5- to 7-day intervals. 48-hr reentry.
    • Kocide 2000 at 1 to 2.25 lb/A or Kocide 3000 at 0.5 to 1.25 lb/A on 5- to 7-day intervals. 48-hr reentry.
    • Liqui-Cop at 2 to 3 teaspoons/gal water. H
    • ManKocide at 2 to 3 lb/A on 7- to 10- day intervals. Under moderate to severe disease pressure use the higher rate on 5- to 7-day intervals. Do not apply within 5 days of harvest. 48-hr reentry.
    • Nu-Cop 50 WP at 1.5 to 3 lb/A. Do not apply within 1 day of harvest. 48-hr reentry. O
    • Previsto at 0.75 to 2 quarts/A on 5- to 7-day intervals. 48-hr reentry. O
  • Seed treatments may help if affected seed is used.
    • Hot water treatments with various chemicals (calcium propionate at 4.4 oz/gal water; acidic cupric acetate at 6.7 oz/gal water) for 20 minutes at 122°F reduced the frequency of disease in cucumber seedlings, but did not completely eliminate the pathogen. Hot water alone is not effective.

Cucurbit Angular Leaf Spot – Managing Angular Leaf Spot Of Cucurbits

Cucurbits with angular leaf spot may give you a smaller harvest. This bacterial infection affects cucumbers, zucchini, and melons, and causes angular lesions on leaves and thrives in warm, humid conditions. You can take measures to prevent this infection and to manage it if you see signs in your garden.

What is Angular Leaf Spot?

Angular leaf spot is a viral infection that affects cucurbit plants. The offending bacterium is called Pseudomonas syringae. The infection may take hold in any cucurbit, but it is most common in cucumbers, honeydew melons, and zucchini. Other melons, squashes, and pumpkins may be infected, but this is less common.

The conditions in which the infection thrives are humid and moist. It is most likely to spread after a big rain or with the use of overhead irrigation. During warm, rainy weather in the summer is when cucurbit angular leaf spot is most likely to take hold.

Signs of Cucurbit Angular Leaf Spot

The infection begins with lesions on the leaves that are water soaked. They will then turn gray to brown in color and are limited by veins in the leaves, hence the angular description and appearance of the lesions.

When the leaves dry out, the affected leaf tissue crumbles and leaves an angular hole in the leaf. This leaves the plant looking tattered. Lesions may grow on the fruits as well, but these are usually superficial.

Angular Leaf Spot Control

Try cultural control for angular leaf spot of cucurbits before trying chemicals to eradicate the infection. Before you ever put any cucurbits in your garden, look for varieties that are resistant to angular leaf spot; many are available.

How you water your garden also makes a difference. Instead of watering overhead, use drip irrigation.

Crop rotation also helps. Rotate cucurbits with other vegetables that are not susceptible to the infection each year. If you have signs of infection in your cucumbers this year, remove the affected foliage and dispose of it, but don’t add it to your compost. You can also till the leaf litter deep into the soil to help it break down.

If you can’t seem to shake the infection, try a bactericide. An early infection may respond to copper sprays.

Angular Leaf Spot of Cucurbits

Hosts

Angular leaf spot, caused by the bacterium Pseudomonas syringae pv. lachrymans, is most severe on cucumber, zucchini squash, and honeydew melon but also can infect muskmelon, cantaloupe, watermelon, other squashes, pumpkin, and various gourds.

Symptoms

On cucumber, leaf symptoms appear as small, round to irregularly shaped, water-soaked lesions. The spots expand until they are limited by larger veins, which give the spots an angular appearance. Under humid conditions, water-soaked spots are covered by a white exudate, which eventually dries to form a thin, white crust on or adjacent to the spot on the underleaf surface. As the spots dry, they shrink and tear away from the healthy tissue, leaving large, irregular holes and giving the leaf a ragged and yellowish appearance. Squash and watermelon leaf lesions are more variable in size and are surrounded by yellow halos. The nearly circular, water-soaked spots on ripening fruit are much smaller than those on the foliage. Similar symptoms develop on musk melon leaves. These lesions eventually become chalky white and may crack open, allowing secondary fungi and bacteria to invade and cause a slimy, foul-smelling fruit rot.

Life Cycle

The Pseudomonas bacterium is a seedborne pathogen. In addition, the pathogen can overwinter in infested crop residues. This disease is widespread and particularly damaging in Illinois after extended and frequent summer rains, especially when temperatures are between 75 and 82 F. Two weeks of dry weather will stop disease development.

Management

Plant certified, pathogen-free seed produced in arid western locations. Resistant cucumber varieties are available. Do not grow cucurbits in the same field more than once every three or four years and avoid excessive nitrogen fertilization. Limit the use of overhead irrigation and avoid cultivating, harvesting, or otherwise handling plants when they are wet. Whereever feasible, cleanly plow under or collect and burn crop debris immediately after harvest. Apply a recommended bactericide at first sign of disease. Tank-mix the recommended bactericide with effective fungicides to protect the plants against fungal diseases.

Filed under plants: Vegetables

Filed under problems: Bacterial Disease

More information is available on Hort Answers.

Cucumber With Holes: What Causes Holes In Cucumbers

Nothing’s more disappointing than cucumbers with holes. Picking a cucumber with holes in it is a fairly common problem. What causes holes in cucumber fruit and how can they be prevented? Read on to find out.

What Causes Holes in Cucumbers?

Some cucumbers are almost hollow inside, which is usually due to improper irrigation or a lack of water. However, a cucumber with holes riddling it is most probably due to an insect of some kind.

Slugs

In my neck of the woods, the Pacific Northwest, the most likely culprit for cucumber holes may be slugs. These guys will eat almost anything and will drill holes through both green and ripe fruit. Sprinkling some slug bait around the plants, however, will likely keep them away from your cucumber plants.

Cucumber Beetles

As the name suggests, cucumber beetles can be very damaging to not only cucumber but other cucurbits such as melons, pumpkins and squash. Cucumber beetles have no preference and will ravage all parts of the plant from foliage to flowers to fruit. They are found throughout the growing season (June-September), but are more likely to cause scarring rather than outright cucumber holes.

Additionally, cucumber beetles transmit bacterial wilt in cucumbers. Bacterial wilt overwinters in the intestines of the pests and is then transmitted from plant to plant as the beetle feeds. Some new varieties of cucurbits have resistance to this disease.

There are several types of cucumber beetle. The spotted cucumber beetle is yellowish green with 11 black dots on its back and a black head with black antennae. The yellow striped cucumber beetle is 1/5-inch long with three black stripes on the tope wings. Lastly, the banded cucumber beetle has yellowish-green stripes that run across the wings.

Handpicking any of these pests is time consuming but effective. Otherwise, the use of fabric row covers is an effective barrier between the pests and plants. Keep the garden free of weeds so the beetles have fewer places to hide. There are also some predatory insects that may be able to aid in the eradication of the beetles. An application of Neem oil or Pyrethrin can eradicate the pests, as well as a number of chemical pesticides.

Pickleworms

Lastly, pickleworms may be the cause of cucumbers with holes. Pickleworms attack most cucurbits — cucumbers, cantaloupes, summer squash and pumpkins may all be severely damaged by the pickleworms’ voracious appetite. Pickleworms aren’t picky and will tunnel through not only fruit, but flowers, buds and stems. Damaged fruit is not edible.

In warmer regions, pickleworms overwinter while in colder areas, the pests freeze during the winter. They go through a complete cycle of egg, larva, pupa and adult. Eggs are irregular in shape and look something like grains of sand. They are laid on leaves in small batches and hatch in three to four days.

The resulting larvae feed on buds, blossoms and tender foliage before they begin on fruit. These brown headed caterpillars molt four times. At the last molt, the caterpillar loses its reddish-brown spots and becomes entirely green or copper in color. It then stops feeding and spins a cocoon to pupate. Pupae are usually found in a curled or rolled leaf and emerge as adults in seven to 10 days as brownish-yellow moths with a hint of purple.

Choose early maturing varieties and plant as soon as possible before the pickleworm population explodes. To control the populations, also destroy any damaged fruit and squash any rolled sections of leaves that contain pupae. Some less toxic or natural controls include Bacillus thuringiensis, Pyrethrin, Neem oil extract and Spinosad as well as other chemical pesticides.

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