NC State and USDA Cucumber Disease Handbook

Scab (Cladosporium cucumerinum)


Scab, Cladosporium rot, spot rot, spotting of cucumber, leaf blight, fruit blight, gummosis.


Cladosporium cucumerinum (Ellis and Arth.)

Culture Description:

On potato dextrose agar (PDA) white when young, but greenish to black with age.

Microscopic Description:


Septate, hyaline, and pigmented.


Conidia mostly one celled, some septate; colored; oblong (4.1-5 x 15.2-18.8 µm); borne on short branched dark conidiophores.


P. H. Williams; Plant Pathology Department; University of Wisconsin; 1630 Linden Drive; Madison, WI 53706.

Relative Stability:

No races reported. No loss of pathogenicity after 8 years of periodic transfer on PDA.


Sectoring occurs to give non-sporulating culture.

Storage and Retrieval:

Store at 4 C on PDA for 3 months or on PDA under sterile mineral oil for 1 year. For retrieval aseptically remove a piece of mycelium, place on PDA, and incubate at 20°C.

Inoculum Increase:

Add a few ml of sterile distilled water to a PDA slant culture. Scrape with a sterile bacteriological loop to dislodge spores. Aseptically remove a loop full of spores and streak over entire surface of PDA slant. Incubate at 25°C.

Inoculum Preparation:

Add a few ml of distilled water to 3-5 day old scab culture grown on a PDA slant. Scrape with sterile bacteriological loop to dislodge spores. Filter through a single layer of cheesecloth to remove mycelial fragments.


Method 1) Count spores with a hemacytometer. Adjust spore concentration to 4 x 105 spores/ml with distilled water. Method 2) County spores. Adjust concentration to 2 x 105 spores/ml. Method 3) Dilute with distilled water until spore suspension is light green in color when held to the light. Check percent germination of spore suspension on water agar after 24 hr.

Inoculum Distribution and Delivery:

Method 1) Spray hypocotyl of plant in cotyledon stage with inoculum using an airbrush. Method 2) Using a Pasteur pipette place a .01-.03 ml droplet of inoculum on cotyledon when cotyledons are just expanded. Inoculating cotyledons beyond this stage may give a resistant reaction on susceptible plants. Method 3) Spray inoculum on growing point and young leaves of plants in cotyledon stage up to fifth leaf stage. Some growing points of resistant plants may be damaged using this method. All seedlings should be marked, by punching the tip of the cotyledon with a Pasteur pipette, at inoculation so late germinating, uninoculated seedlings can be distinguished from resistant plants.


Cucumis sativus L., cucumber.

Source of Resistance:

Wisconsin SMR 18.

Differentials – Controls:

Susceptible check ‘Straight Eight’. Resistant check ‘Wisconsin SMR 18’.

Growth of Host:

Cucumber seeds are sown in steam sterilized coarse grade vermiculite in wooden flats (52 x 36 x 7 cm). Each flat contains 10 rows, 25 seeds/row. Resistant and susceptible checks are sown in row 6. The flats are placed on a heated germination bench. Vermiculite temperatures of 32°C insure rapid and uniform germination. Newspaper on top of the plat prevents cooling by evaporation. Newspaper is removed when seeds germinate. If inoculating plants in the true leaf stage, transplant 2 wk old seedlings to steam sterilized soil in 4″ plastic pots. Soil composed of sand:peat:field soil:field compost:perlite (1:1:1:1:1). Fertilize plants in 4″ pots once/wk.

Tissue Age:

Methods 1 and 2) Inoculate plants when cotyledons are just expanded. Method 3) Spray inoculum on plants in cotyledon stage up to fifth leaf stage.

Postinoculation Environment:

Incubate plants at 17-20°C in the dark for 48 hr at 100% relative humidity. If leaves become water soaked, leaf tissue on older plants may collapse and resistant plants appear susceptible. After incubation grow plants at 25°C. Warmer temperatures can cause a resistant reaction on susceptible plants.

Disease Response:

Plants are rated as susceptible or resistant 5-7 days after inoculation. Method 1) The hypocotyl of susceptible plants is girdled or has sunken necrotic lesions. Resistant plants show no reaction or, rarely, a faint water soaked lesion. Method 2) A sunken lesion develops on the cotyledon of susceptible plants. Resistant plants show no reaction or a glossy spot where inoculum was applied. Method 3) The growing point of susceptible plants is killed and young leaves have necrotic lesions. Resistant plants remain healthy, though some damage to the growing point may occur.

Multiple Inoculation:

Methods 1 and 2) Simultaneous inoculation with anthracnose, downy mildew, angular leaf spot, or bacterial wilt. Subsequent inoculation with cucumber mosaic virus (CMV) and powdery mildew. With method 2 scab resistant seedlings inoculated with C. cucumerinum and Colletotrichum lagenarium (anthracnose) had fewer lesions and less disease severity than seedlings inoculated with C. lagenarium alone. Spray method: Previous inoculation with anthracnose, downy mildew, angular leaf spot, bacterial wilt, CMV, and powdery mildew.

Saving Host:

Method 2. Both resistant and susceptible seedlings can be transplanted to steam sterilized soil. Methods 1 and 2. Only resistant plants survive.

Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin