Watermelon Screening Method for Gummy Stem Blight Resistance

by Gabe Gusmini and Todd C. Wehner
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7609

After running many experiments, we would recommend the following:

  • cultures grown for 2 to 3 weeks on potato dextrose agar under artificial light for 12 hours per day
  • use of isolate mixtures to ensure pathogenicity
  • constant re-isolation of new isolates from diseased plants in the field
  • suspension of spores in water for spray application
  • a spore concentration of 500,000 spores/ml
  • addition of surfactant to the suspension ([Tween 80] = 0.06 g/l)
  • use of high relative humidity immediately following inoculation (with the presence of free-standing water on the leaves of test plants)
  • pre-inoculation damage of the leaves

Environmental variation requires that many treatment combinations be used to evaluate resistance to gummy stem blight. Treatments can include years, seasons (or planting dates), replications, plants, and test types (field and greenhouse).

Field inoculation method: leaf damage, followed by inoculation (JPEG image)

If the objective were to develop resistant inbreds, we would recommend that the experiment have:

  • 1 year (for rapid progress)
  • 1 to 3 seasons (spring, summer, fall)
  • 3 to 5 replications
  • 6 plants/plot
  • 2 test types (greenhouse, field)

Inoculum preparation

Small scale production of spores in Petri plates
Large scale production of spores in Nalgene boxes
Spore suspension ready for dilution and inoculation

Types of humidity chambers for greenhouse test

Inexpensive and temporary type
Inexpensive and semi-permanent type
Expensive and permanent type