Sweet Corn Disease Nursery

Stewart's Bacterial Wilt

Erwinia stewartii

Rating Stewart's Wilt

Following inoculation, plants are rated for Stewart's wilt symptoms on a 1 to 9 scale, where 1 = no appreciable spread of symptoms from wounds; 2 = limited water-soaking, chlorosis or necrosis within 3 cm of wounds; 3 = limited spread from wounds, chlorosis or necrosis predominantly towards tips of leaves; 4 = abundant spread from wounds, chlorosis or necrosis towards both ends of leaves; 5 = minimal systemic infection, a few symptomatic streaks on non-inoculated leaves; 6 = moderate systemic infection, symptoms on 5-25% of leaf area, minimal stunting; 7 = abundant systemic infection, symptoms on 25-50% of leaf area, distinct stunting; 8 = 50-90% of leaf area symptomatic, severe system infection and stunting; 9 = 90-100% of leaf tissue symptomatic or dead plants. Ratings <3 indicate limited spread of E. stewartii that is indicative of resistant or moderately resistant reactions. Ratings of >5 indicate systemic infection by E. stewartii that is indicative of moderately susceptible or susceptible reactions.

Since Stewart's wilt reactions vary among plants in an experimental unit (usually a 1- or 2- row plot with 10 to 15 plants per row), and it would be very time consuming to give each plant an individual rating, we typicallygive each plot three Stewart's Wilt ratings that are indicative of the variation within the plot. For example, a rating of 2-5-6 indicates that about a third of the plants have symptoms that are about a 2, a third are about a 5, and a third are about a 6; although within the plot there may be a few plants that are 3s, 4s or 7s. Likewise, in a plot rated 3-3-3, most plants have symptoms typical of a 3. For most analyses, we use the mean of the three ratings per plot.

Rating Scale