NC State and USDA Cucumber Disease Handbook
Cucumber Disease Descriptions
Consult your local agricultural Extension Service for the latest recommendations on the use of chemicals. The use of resistant varieties; seed grown in the dry inner valleys of California; prompt destruction of old cucurbit crops, volunteers, and certain weeds; and practicing at least two years, preferably three, rotation between cucumber, watermelon and cantaloupe will avoid many of the disease problems. Viruses survive between cucumber crops on weeds, other crops, and other cucurbits (squash, watermelon, cantaloupe). They are spread from these hosts and between cucumber plants by aphids and beetles. Powdery mildew also survives on similar plants and is spread by wind currents. Scab, anthracnose, gummy stem blight and angular leaf spot survive on infested plant residues, on old plantings and volunteer cucurbits. All except scab may be seed borne. Scab may spread in dew whereas the others require rain to splash or blow the spores. Downy mildew survives on infested plants in subtropical areas and spreads northward in storm currents. Additionally all these diseases may be spread by machinery, personnel, animals or insects moving through the fields, especially when the foliage is wet.
Angular Leaf Spot
Angular Leaf Spot appears on leaves as small water-soaked spots whose increase in size is limited by veins to form an angular shape. The upper surface of the spot becomes tan, and a sticky milky bacterial ooze forms on the lower surface and dries into a white crust. Many of the diseased spots loosen and drop out leaving a shot-hole effect. On stems and fruits the water-soaked areas may also become covered with crusty white exudate. As fruits mature, brown lesions form in the flesh and continue as streaks down to the seed. Is infrequent and is caused by the seed-borne bacterium Pseudomonas syringae pv. lachrymans which can also affect squash and pumpkins. Warm, wet weather may cause stem and leaf spots in mid-season. CONTROL: Follow general management practices; start spray schedule at first appearance; do not enter fields when plants are wet.
Anthracnose is common and is caused by the seed-borne fungus Colletotrichum orbiculare. Most cucurbits are susceptible. Most commercial cucumber varieties have some resistance. Depending on weather, leaf and fruit spotting may occur on young plants, especially in late plantings. CONTROL: Follow general management practices; start spray schedule at first appearance. Infects young tissue and begins on a leaf vein as small yellowish circular spots. Very young leaves may be distorted. The spots gradually become brown and enlarge to 1/4 inch. They remain circular and the center frequently drops out leaving a shot-hole appearance. Stem lesions are shallow, elongate, slightly sunken, and tan in color. Fruit lesions begin as small, circular, water-soaked spots which enlarge to 1/2 inch and become tan. Under moist conditions pinkish spore masses cover these spots.
Bacterial Wilt is caused by the bacterium Erwinia tracheiphila. It overwinters in and is spread by cucumber beetles. Often seen in early to mid-season cucumbers and when temperatures are cool, it occurs on muskmelon and to a lesser extent on squash and pumpkin; watermelons may be immune. CONTROL: Control the cucumber beetles, spray.
Belly Rot is caused by the fungus Rhizoctonia solani, present in agricultural soils. It causes damping-off of many crops including cucumbers. Belly rot is common, but is occurrence and intensity are unpredictable. CONTROL: Avoid problem fields; apply fungicide to soil prior to fruit set; keep harvested cucumbers cool, well ventilated; use chlorine in wash.
Cottony Leak is caused by the fungus Pythium aphanidermatum and others. Present in agricultural soils; it causes damping-off of many crops including cucumbers. Cottony leak is common, but its occurrence and intensity are unpredictable. CONTROL: Avoid problem fields; apply fungicide sprays to soil and foliage prior to fruit set; keep harvested cucumbers cool, well ventilated; use chlorine in wash.
Downy Mildew is caused by the air-borne fungus Pseudoperonospora cubensis. Although serious on late season cantaloupes, pumpkins, squash, watermelon, and other cucurbits, it is seldom a problem on cucumbers because most commercial varieties have adequate resistance. The fungus over-winters in Florida, and each year the spores are blown northward and arrive in North Carolina from May to August, if at all. Typically its arrival coincides with summer rains. CONTROL: Resistant cultivars; start spray schedule at first appearance. Begins as small yellowish areas on the leaves. Early in the morning when moisture has been present overnight, young spots may appear as water-soaked, somewhat angular areas on the lower side. As the lesions enlarge in a somewhat angular manner they change from yellow to brown. On the lower side, purplish spore masses may be seen, and several infected leaves may die in 10-14 days. Downy mildew does not attack stems or fruit.
Gummy Stem Blight
Gummy Stem Blight is a warm weather southern disease. On leaves it causes conspicuous V-shaped brownish lesions. Many times it produces brownish circular spots which rapidly expand into large tan lesions of round, indefinite size. On stems, circular fawn (tan) lesions frequently elongate and turn gray and are present mainly in the nodes. The lesions may exhibit brown gummy exudate on the surface and stems may be girdled. In prolonged rainy periods most of the foliage and vine may rapidly collapse. Fruit lesions appear as small water-soaked areas, similar to anthracnose, which rapidly enlarge to indefinite size. Also, a brown streak may appear in the blossom end of the fruit. Although this disease may attack young tissue, it is primarily a disease of older tissue. Is common and is caused by the seed-borne fungus Didymella bryoniae. All cucurbits are susceptible, and depending on the weather, stem and leafspotting may occur on young plants, especially in late plantings. Badly infected plants may have a gummy exudate from stem lesions. CONTROL: Follow general management practices; start spray schedule at first appearance.
The yellow and green mosaic patterns of cucumber viruses, commonly seen on young leaves near the growing tip, often change to an indistinguishable mottle as the leaves mature. A dwarfing of the vines may become more conspicuous as leaf mottle symptoms tend to disappear. Cucumber mosaic and watermelon mosaic viruses attack cucumbers and produce similar symptoms; however, cucumber mosaic virus does product a distinguishing symptom on fruit. Fruit infected with cucumber mosaic are characterized by a lack of color (“white pickle”) in addition to the mottling and an extensive, bumpy, roughened appearance produced by all the cucurbit viruses. Mosaics are caused by viruses. Depicted is Tomato Ringspot, fairly common in the northwestern part of the state. The virus resides in wild brambles and is transmitted by some nematodes. Most commercial cultivars have excellent resistance to Cucumber and Watermelon Mosaic. Until recently mosaics were rarely seen in the eastern counties. Because Zucchini Yellow Mosaic (closely related to Watermelon Mosaic) may be introduced, it is important to identify all mosaics. Mosaic viruses are transmitted by aphids and related insects. CONTROL: Select resistant cultivars; avoid problem fields; start spray schedule with stylet oil and insecticides at the seedling stage; use aluminum plastic mulch.
Powdery Mildew is caused by two air-borne fungi Erysiphe cichoracearum and Sphaerotheca fuliginea, but is rarely seen since most commercial cucumbers have excellent resistance. Powdery mildew is often serious in dry weather and is the main cause of early dying of cantaloupes, pumpkins, squash, and greenhouse cucumbers. CONTROL: Resistant cultivars; sprays; because of fungicide specificity and possible development of resistance in the fungus, switch fungicides if spraying is not effective. First occurs on stems and leaves as tiny, while superficial sports, becoming covered with powdery spore masses as they enlarge. Under ideal conditions rapid, early, shedding of leaves may occur as entire leaf surface becomes covered with the powdery spores. Fruit is not infected.
This common rot starts as a small water-soaked spot, particularly where fruit is touching the ground, which enlarges into a raised, water filled blister about 1/4 inch in diameter. Further enlargement is very rapid, with the disappearance of the blister symptom coupled with the occurrence of soft, watery, brown rotten tissue covered with a fluffy white mat. This entire process may occur in a 48 hour period.
Root-Knot is caused by Meloidogyne spp. These nematodes are usually present in agricultural fields and affect many crops. They are especially damaging on plants growing in light, sandy soils when counts are high. CONTROL: Promptly disk all crops after harvest, treat soil with a fumigant nematicide when warranted by assay report.
Scab (Spot Rot)
Scab (Spot Rot): first appears on leaves as small, circular to angular water-soaked brownish spots difficult to distinguish from angular leaf spot. Scab lesions are not covered with a crusty white exudate. Under high humidity leaf sporulation occurs on these spots as an olivaceous or velvety mat. On young fruit a gummy brown exudate occurs on the surface of the water-soaked areas, and as the spot spreads, the tissue sinks and dries to form a tan scab. Brown streaks do not extend into the flesh as for angular leaf spot. Is caused by the seed-borne fungus Cladosporium cucumerinum which also affects muskmelons. It is rare because most commercial cucumbers have excellent resistance. Cool night temperature and high humidity is required. In the active phase, dark, olive-green, velvety mold covers the spot. Leaves, fruit, and young terminals may be affected. CONTROL: Use resistant varieties, spray at first appearance.
Target Spot is caused by the air-borne fungus Corynespora cassiicola which attacks many different plants. It is more common on cucumbers in greenhouses than in fields. CONTROL: Follow general management practices; start spray schedule at first appearance.