University of Illinois Research on Wheat Scab Resistance

by Frederic L. Kolb, Larry Boze, and Norman Smith, Department of Crop Sciences, University of Illinois

• Introduction
• Procedures
• Findings
• Summary Points
• Table 1

Introduction:

Scab, or Fusarium head blight, significantly reduces yield and grain quality of wheat. In the past, plowing helped to control scab by reducing the number of spores available to infect wheat. With increasing conservation tillage more Fusarium spores survive on crop residue, and damage due to scab is increasing. When environmental conditions favor infection, severe epidemics result. Consequently, an immense need exists for varieties with scab resistance. Therefore, development of scab resistant germplasm and varieties is a major research emphasis in the University of Illinois wheat breeding program. The long-term objective is to develop soft red winter wheat varieties with excellent resistance to scab combined with high yield potential, acceptable winter hardiness and milling and baking quality, and resistance to other diseases. Our short-term objectives are: 1) to combine genes for resistance to scab from diverse sources; 2) to evaluate breeding lines produced from crosses and identify those with resistance to scab; and 3) to identify molecular markers associated with genes for resistance to scab

Procedures:

In 1998, we evaluated scab resistance using both greenhouse and field procedures with misting systems and inoculation. Each year breeding lines are evaluated using a scab resistance evaluation field nursery at the Crops Sciences Research and Education Center. To promote disease development in the field nursery we used a mist irrigation system and inoculated with scab infected seed. Infected wheat seed was scattered on the ground in the nursery two times beginning about one month before flowering. The nursery was mist irrigated each day (one hour in the morning and one hour in the afternoon) unless it rained. Because infection takes place at, or just after, flowering, mist irrigation began when the earliest plants began flowering and continued until all plants had flowered. Scab symptoms were rated in the field nursery by estimating incidence (percentage of the heads with scab symptoms) in each row and by evaluating severity (percentage of the spikelets in a head with symptoms) on 20 random heads from each line (10 heads in each of two replications). A scab index was determined by multiplying the percentage of heads that were infected by the average percentage of spikelets that show symptoms in the infected heads. Seed was harvested from each entry in all experiments, and kernel quality was evaluated after careful cleaning. Seed quality was evaluated on a 0 to 9 scale where entries with a 0 score had essentially no scabby or shriveled seeds, and entries with a 9 score were nearly all scabby and shriveled seeds. This is a very severe test, because the inoculation and the misting strongly favor disease development. We had very high scab incidence in the 1998 scab evaluation field nursery.

Material evaluated included germplasm reported to be resistant, current varieties, and experimental lines. In 1998, we evaluated about 750 experimental breeding lines and varieties, and 7 segregating F₄ populations in the misted, inoculated field nursery.

Findings:

Many of the breeding lines were susceptible to scab, but variability for scab resistance exists in the soft red winter wheat germplasm. Based on field symptoms and evaluation of kernels, some experimental lines with partial scab resistance were tentatively identified. About 500 plants were selected from the segregating populations in the field. These materials will be evaluated further to determine the level of resistance. Some recently released varieties have better scab resistance than older varieties (Table 1.). Experimental breeding lines with Ning 7840 (a spring wheat with scab resistance from China) in the parentage exhibited the best scab resistance. Unfortunately, many of these breeding lines also have undesirable traits such as tallness or susceptibility to lodging and leaf rust.

Summary Points: What We Learned

• Genetic variability for scab resistance is available, both from unadapted sources of resistance and from existing soft red winter wheat germplasm.
• The techniques we are now using have greatly improved our capability to select for scab resistance.
• Additional research and time are required to combine scab resistance with the many other traits that are required in a successful variety.
• Many soft red winter wheat breeding programs are emphasizing scab resistance. Recently developed varieties, such as Patton, Roane, Ernie, and Goldfield have better scab resistance than many older varieties; however, the level of resistance is generally not sufficient to provide protection under high disease pressure.

Table 1. Scab ratings of wheat varieties in the inoculated, misted nursery at Urbana, 1997-98

Scab Index: 0 = no symptoms to 100 = all heads infected and severe spread within every head.

Seed Rating: 0 = excellent, no scabby seeds to 9 = all seeds scabby and shriveled.