Fruit Rots of Pickling and Slicing Cucumbers
Vegetable Disease Information Note 1 (VDIN-001)
Gerald J. Holmes, Extension Plant Pathologist

Belly Rot Cottony Leak Anthracnose Scab Black Rot Bacterial Spot

Fruit rots, caused by soil-borne fungi or bacteria, are major diseases of pickling and slicing cucumbers in North Carolina and the southeastern United States. Their occurrence is unpredictable because of changes in weather and soil conditions. Losses occur in the field and often continue after harvest. An integrated plant disease control program is necessary to reduce losses from these rots. Each step provides an increment of aid in managing fruit rots of cucumbers.

Drainage. Use cultural practices which will reduce standing water, humidity, and improve field drainage. Land leveling will reduce standing water. Slightly sloped beds and fields are recommended for good drainage. Select fields with a sandy loam soil that will provide good drainage.

Land Preparation. Plow so that the top 2 to 4 inches of soil is completely turned under (see control section Information Note 9). Disking or similar tools that mix the top few inches of soil will not reduce inoculum levels and subsequent disease. If hardpan layers are present and cause slow drainage in soil, use a chisel plow during land preparation.

Plant Density. Plant populations should be limited to between 20,000 and 25,000 plants per acre for river bottomland and about 25,000 plants for sandy, well-drained soils. Consider the vine habits of the cultivar; a dense canopy usually results in more fruit rot because it promotes conditions that are conducive to disease. Slightly higher plant populations can be utilized if a sparse-canopy cultivar is chosen. The timing and method of irrigation should be such that long periods of excess moisture on the soil surface are minimized. For example, one inch of water on a weekly cycle is preferred to half an inch on a 3- to 4-day cycle. High humidity and moisture favor fruit rots and should be considered when devising the irrigation schedule.

Rotation. Rotate cucumber with grasses or small grains. Intensively cropping cucumbers on the same fields will encourage fruit rots and other diseases. The longer the rotation out of cucumbers, the better.

Fertilization. Excess nitrogen fertilizer promotes lush growth of vines, creating a dense canopy of succulent tissue. This type of rank growth provides conditions conducive to disease development. For this reason, over fertilization should be avoided.

Fungicides. Azoxystrobin (Quadris), chlorothalonil (Bravo) and thiophanate-methyl (Topsin M) are labeled for control of belly rot. Products containing mefenoxam (Ridomil Gold) provide some protection from cottony leak (Pythium rot). These fungicides will give disappointing results under heavy disease pressure (consult table 6-18 in NC Agricultural Chemicals Manual for fungicide efficacy ratings).

Figure 1. Chilling injury.

Postharvest control strategies. Harvested fruit should be kept as dry and cool as possible. Harvested cucumbers in containers will generate heat. Thus, these containers should be kept out of the sun and ventilated with fans when there is no wind. Fruit should not be washed until ready for processing or brining. If they must be washed, add 50 ppm of chlorine in the wash water. Harvesting and handling fruit in a manner to minimize bruising or wounding will reduce losses. If fresh fruit needs to be transported long distances, it should be kept at 45 to 50 degrees F and transported as rapidly as possible. Chilling injury will result if fruit is held at temperatures below 45 degrees (Figure 1).

Diseases

Belly Rot

Occurrence and Importance. Belly rot (soil rot, Rhizoctonia rot) is one of the most common fruit rots of cucumbers in North Carolina. Losses vary from year to year with a loss of 5 to 10 percent in warm, humid weather. Fruit becomes infected where they contact the soil in the field. Fruit that are apparently healthy, but have already been infected, can develop severe symptoms within 24 hours.

Cause. Belly rot is caused by the cosmopolitan soil-inhabiting fungus Rhizoctonia solani.

Figure 2. Water-soaked decay caused by R. solani. Figure 3. Water-soaked areas turn scabby and cracked.

Symptoms. Very small cucumbers show yellowish-brown superficial discoloration. Large fruit have dark brown water-soaked decay most often on the side of the fruit in contact with the soil (Figure 2). If the lesion is allowed to dry (just a few hours) the watersoaked areas turn scabby and cracked (Figure 3).

Disease Development. The fruit can develop symptoms 24 hours after coming in contact with the fungus and the entire fruit can rot in 72 hours. Temperatures below 50 degrees F retard disease development during transit and storage.

Cottony Leak

Occurrence and importance. Cottony leak (Pythium fruit rot) causes serious losses of cucumber in North Carolina and other southeastern states. Infection occurs in the field and decay progresses rapidly. Losses occur during wet weather over a wide range of temperatures. Often it is worse after a drought. The Pythium species, which cause this disease, remain in the soil and
attack a number of different host plants.

Figure 4. Pythium sp. growing on cucumber fruit.

Symptoms. The first symptoms on the fruit are soft, dark green, water soaked lesions or spots. Later, as it penetrates the tissues of the fruit, water is liberated in large quantities. In the humid atmosphere of the vine canopy (or the containers used during transit) a very luxuriant, white, cottony fungal growth completely covers the fruit (Figure 4). The cottony growth may become flattened and matted over the fruit.

Disease Development. Pythium is spread by microscopic, swimming spores or by contact between diseased and healthy fruit tissues. Visible wounds are not necessary for entry to the fruit. Temperatures below 60 degrees F retard Pythium growth and rot is slowed considerably. Cucumbers should be cooled to and held at that temperature until processed.

Anthracnose

This disease is characterized by sunken lesions colored by salmon or pink spore masses, and is caused by the fungus Colletotrichum orbiculare. This occurs only when anthracnose susceptible cultivars are planted. For more information on anthracnose see Plant Pathology Information Note No. 11.

Scab

Caused by the fungus Cladosporium cucumerium, this disease is characterized by small grayish, slightly sunken spots on the fruit. Usually this disease occurs only in the mountains of western North Carolina on susceptible cultivars.

Black Rot (Gummy stem blight)

Caused by the fungus Didymella bryoniae, small water-soaked spots develop that gradually enlarge and darken. Despite the fact that this is a serious foliar disease of cucumber in North Carolina, very little loss occurs due to fruit rot. See Plant Pathology Information Note No. 8 for more information.

Bacterial Spot (Angular leaf spot)

This disease is caused by bacterium Pseudomonas syringae pv. lachrymans, and causes minute, circular water-soaked lesions that later become sunken and chalky-white in color. This fruit rot phase is rarely seen and losses are very low. Most modern cultivars possess good resistance to this disease.

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