Spots on leaves begin as small yellowish or water-soaked areas that enlarge rapidly and turn black on watermelon. Petioles may be attacked causing defoliation. Elongate lesions similar to those on leaves are present on stems; these lesions, together with destruction of the foliage, may kill the whole vine. Dark, sunken, circular spots develop on the fruit. When the fruit pedicel is attacked, young fruits darken, shrivel, and die resulting in fields with vines having small black fruit.
Source of primary inoculum:
Fungal structures surviving on crop debris and contaminated seed.
Source of Secondary inoculum:
Spores that are produced on infected plants.
The fungus overwinters in infected plant debris or on seed. In the spring, spores of the fungus are dispersed by splashing water and infect the foliage and other plant parts. The infection process requires free water on the leaf surface and temperatures between 20-32 C. Infection proceeds rapidly during warm wet weather.
Disease free seed, resistant cultivars, crop rotation, and fungicide application..
On potato dextrose agar (PDA) mycelium is submerged, colorless, becomes dark with age. Acervuli consist of orange masses of conidia.
Mycelium: septate; hyaline when young, dark when old. Spores: conidia are hyaline; oblong to ovate (4-6 x 9-13 microns); piled up in a slimy orange mass; germinate to produce dark, thick walled spherical appressoria.
P. H. Williams; Plant Pathology Department; University of Wisconsin; 1630 Linden Drive; Madison, WI 53706. Races 1 and 2 available. The American Type Culture Collection; 12301 Parklawn Drive; Rockville, MD 20852.
There are three pathogenic races of C. lagenarium.Races 1, 2, and 3.
After repeated mass transfer of spores, cultures of race 1 sporulate poorly. Retrieve a culture from storage or ‘single spore’ the low sporulating culture. Single spores segregate mycelial type and sporulating type.
Aseptically remove a loop full of spores from a 5-10 day old culture and streak over entire surface of a baby food green bean agar (BFGRA) slant. Incubate at 24°C.
Add a few ml of distilled water to a 6-9 day old culture and shake vigorously for 10 seconds on a Vortex mixer to dislodge the spores. Wash the spores by centrifuging at 3,000 rpm for 10 min. Discard the supernatant. Resuspend the pellet in distilled water.
Count spores with a hemacytometer. Adjust spore concentration to 8 X 10E5 spores/ml with distilled water.
Inoculum Distribution and Delivery:
Method 1) Spray inoculum on upper surface of one cotyledon with a painter’s air brush at 5-10 psi air pressure. Method 2) Using a Pasteur pipette place a .01-.03 ml droplet of inoculum, 1/10 dilution (8 X 104 spores/ml), on the upper surface of one cotyledon. Spread the droplet gently and uniformly over the cotyledon using a fingertip.
Citrullus lanatus, watermelon.
Source of resistance:
PI 299379 and PI 189225 are reported to resistant to race 2.
Growth of Host:
Watermelon seeds are sown in steam sterilized coarse grade vermiculite in wood 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 is removed when seeds germinate.
Inoculate cotyledons 3-4 days after emergence when cotyledons are expanded.
Incubate plants in the dark for 48 hr at 20°C and 100% relative humidity. Subsequently place plants in greenhouse 24-32°C.
Plants are rated as susceptible, intermediate or resistant 6-8 days after inoculation. Lesions on susceptible plants are brown with chlorotic halos. Lesions enlarge and coalesce. Lesions on resistant plants are tan, surrounding tissue remains green. Lesions remain small and localized. On intermediate plants a yellow-green halo surrounds the lesion and tissue around the lesion often puckers.
Resistant, intermediate, and susceptible plants can be transplanted to steam sterilized soil.
Procedures Developed by:
Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin