NC State Watermelon Disease Handbook

Fusarium Wilt (Fusarium oxysporum f.sp. niveum)

Disease:

F. oxysporum f. sp. niveum (E. F. Sm.) Snyd.

Cause (pathogen):

Fusarium oxysporum f.sp. niveum

Symtoms:

The disease causes wilting and decline, which may occur in entire plants or in individual runners. Most commonly, the entire plant wilts quickly, without yellowing, and then turns brown and dies. A longitudinal cut in the lower stem of infected plants will reveal yellow, orange, or brownish streaks in the vascular tissue.

Source of primary inoculum:

Resilient spores that survive in soils for indefinite periods of time.

Source of Secondary inoculum:

none

Spread:

Mechanical spread with soil on farm implements from year to year.

Disease cycle:

Fusarium wilt of watermelon is a soilborne disease. The fungus attacks seed, seedlings, and roots. The fungus spreads from roots through the vascular system as microconidia reaching all parts of the plant. The fungus is persistent in the soil and is spread long distances on infected seed. Once a host is infected, the fungus increases dramatically. Both macroconidia and chlamydospores are produced on the surface of tissue killed by the organism.

Control:

Watermelon varieties with excellent resistance to Fusarium wilt are available. There is no chemical control of fusarium wilt of watermelon. Long rotations of non-cucurbit crops will help to slowly reduce Fusarium populations in soil. Substantial losses will occur if susceptible varieties are planted into fields with a history of the disease, even though a long rotation had been employed. Standard three- to four-year rotations also are recommended even if resistant varieties are used.

Culture Description:

On potato dextrose agar (PKA white mycelium, purple pigment usually develops with age.

Microscopic Description:

Mycelium:

Septate.

Spores:

Macroconidia are crescent shaped, multiseptate, fine, and well pointed; Microconidia are single celled and oval; Conidiophore are very short bearing microconidia in false heads; Chlamydospores are the resting structures having thick cell wall.

Source:

American Type Culture Collection; 12301 Parklawn Drive; Rockville, MD 20852. Races 1 through 3 available. P. H. Williams; Plant Pathology Department; University of Wisconsin; 1630 Linden Drive; Madison, WI 53706. Race 2.

Relative Stability:

Three races reported. Among them, race 2 is highly aggressive one.Probably loses pathogenicity after periodic transfer on PDA.

Variants:

No information.

Storage and Retrieval:

Store in sterile soil at 4°C for several years. Store on PDA or PDA under sterile mineral oil for 3 months. For retrieval aseptically remove soil or mycelium, place on PDA, and incubate at 24°C.

Inoculum Increase:

Place a piece of mycelium in 50 ml of potato dextrose broth (PDB) in a 125 ml Erlenmeyer flask. Shake on a rotary shaker 26-30°C for 3-6 days.

Inoculum Preparation:

Comminute fungus and PDB in Waring blender at low speed for 30 sec. 2 times. Centrifuge at 3,000 rpm for 10 min. to wash the spores. Discard supernatant. Resuspend pellet in distilled water.

Quantification:

Count spores with a hemacytometer. Check percent germination of spore suspension on water agar after 24 hours.

Inoculum Distribution and Delivery:

Fill metal pan (50 x 29 x 11 cm) with silica sand. Weigh sand. Need 105 spores per gram sand. Calculate ml of concentrated inoculum needed per pan. Mix inoculum with deionized water to 1,500 ml volume. Add to sand. Mix thoroughly in a large mixing pan. Return sand to metal pan. Make 8 1.5 cm furrows. Plant 20 seeds per row. Row 5 contains resistant and susceptible checks.

Host:

Citrullus lanatus, watermelon.

Source of Resistance:

Susceptible Check:

Black Diamond or Sugar Baby.

Growth of Host:

See inoculum distribution and delivery section. Water with tap water or fertilizer daily. Photoperiod 12 hr. light, 12 hr. dark.

Tissue Age:

Seeds are sown in infested sand. See inoculum distribution and delivery section.

Postinoculation Environment:

Place pans in temperature tanks. Sand temperature 28°C. No light first 2 days to prevent drying.

Disease Response:

Plants are rated on a scale from 0 to 9 three weeks after inoculation.

0 = no symptoms
1 = hypocotyl browning, no wilt, no stunting
3 = cotyledon lesion, no wilt, no stunting
5 = slight wilt, stunted
7 = severe wilt, stunted
9 = dead

Plants rated 0 are classified as resistant, 1 or 3 as intermediate, 5, 7, or 9 as susceptible.

Multiple Inoculation:

No information. Experiments planned.

Saving Host:

Using this method resistant and possibly intermediate plants can be transplanted to steam sterilized soil.

Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin
10-11-82