Bacterial Wilt (Erwinia tracheiphila)
Erwinia tracheiphila (E. F. Smith) Holland.
On agar media grayish white to cream, circular, smooth and glistening bacterial colonies. Infested stems exude sticky, stringy bacterial slime when cut.
Gram negative, non-sporeforming, capsulate rods (0.5-0.7 x 1.2-2.5 µm); motile with peritrichous flagella.
P. H. Williams; Plant Pathology Department; 1630 Linden Drive; Madison, WI 53706.
Highly unstable in culture. Loses virulence and viability quickly. No evidence of change in pathogenicity in nature.
Storage and Retrieval:
The bacteria can be stored in excised, infected hypocotyls, petioles, and stems at -17°C for 1 year. To retrieve, grind frozen tissue and use for inoculum.
Bacteria are maintained in susceptible host tissue, fresh or frozen. To increase inoculum, inoculate each leaf of a susceptible plant as for routine inoculation (see inoculum preparation, quantification, and inoculum distribution and delivery sections). Grow inoculated plant in greenhouse 24-28°C. Fertilize once/week. When petioles are drooping due to bacterial wilt, check a petiole for stringy, sticky bacterial slime. When present, the petioles and stems can be harvested for inoculum. Use tissue immediately or freeze.
Infected hypocotyl, petiole, or stem tissue, fresh or frozen, is ground using a mortar and pestle.
Check tissue for slime. If very slimy, a small amount of tap water can be added to increase inoculum. Check again for slime. If tissue is not slimy, discard.
Inoculum Distribution and Delivery:
Place some ground tissue in the needles of a flower frog, 1 cm diameter, 20 needles arranged in concentric rings. Dip the needles in inoculum. Puncture the cucumber cotyledon or leaf twice with the flower frog needles to insure penetration of bacteria. Dip the needles in inoculum before wounding each plant.
Cucumis sativus L., cucumber.
Source of Resistance:
PIs reported to contain some resistant plants are 200815, 200818, and 222187.
Differentials – Controls:
Susceptible check Straight Eight. Resistant check WI 1589 from Dr. C. E. Peterson.
Growth of Host:
Cucumber seeds are sown in steam sterilized coarse grade vermiculite in wooden flats (52 x 26 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 flat prevents cooling by evaporation. Newspaper is removed when seeds germinate. If inoculating plants in the leaf stage, transplant 2 week old seedlings to steam sterilized soil in 4″ plastic pots. When plants are 4 weeks old, transplant to 8″ clay pots. Soil composed of sand:peat:field soil: field compost:perlite (1:1:1:1:1). Fertilize plants in 4″ and 8″ pots once/week.
Inoculate young tissue, either cotyledons or leaves, of any size plant.
Incubate in the greenhouse at 24-28°C. High humidity is not required, but plants may be placed at 100% relative humidity and 25°C for 48 hours.
Plants are rated as susceptible or resistant 5-7 days after inoculation. Susceptible plants wilt at point of inoculation. The wilt progresses along the stem. Resistant plants do not wilt.
Simultaneous inoculation in the cotyledon stage with scab, anthracnose, angular leaf spot, or downy mildew. Subsequent inoculation with cucumber mosaic virus (CMV) and powdery mildew.
Resistant plants can be transplanted to steam sterilized soil. Susceptible plants die. If a desirable plant is presumed to be susceptible, a cutting can be inoculated to check disease reaction. Susceptible plants inoculated at the leaf tip may usually be saved by excising the inoculated leaf at the junction of the petiole and stem as soon as wilting appears.
Paul H. Williams
Mary J. Palmer
Department of Plant Pathology
University of Wisconsin