Historical Durability of Resistance to Wheat Diseases in Kansas

In Kansas, estimated annual losses from wheat diseases averaged 16% from 1976 through 1988; however, losses have declined to 10% in recent years. This decline is mainly due to emphasis on developing cultivars with resistance to important diseases. Data from annual KSU extension publications were used to track disease ratings over time for individual wheat cultivars to eight diseases to draw inferences on the durability of deployed resistance. The duration of durability of resistance to leaf rust has been short for some cultivars but moderate for others. Conversely, the duration of durability of resistance to stem and stripe rusts has been long, although a recent virulence shift in the stripe rust pathogen indicates only moderate duration. Duration of durability of resistance toward tan spot, Septoria tritici blotch, wheat soilborne mosaic, and wheat spindle streak mosaic has been long. Resistance to wheat streak mosaic has been of long duration in some cultivars but others have shown only moderate duration of durability. The effort by breeders to produce cultivars with resistance to diseases in Kansas has resulted in an annual savings of about $58 million. Just as important as the incorporation of resistance into those cultivars is the moderate to long duration of durability of the resistance contributing to effective management of these diseases. Introduction There are about 20 wheat diseases that may be found in Kansas (3). Only 14 of those cause enough loss to warrant control methods including the development of resistant wheat cultivars. Planting resistant cultivars is arguably the best control method for plant diseases. Annual savings to wheat producers in Kansas from planting resistant wheat cultivars has been reported to be about $58 million (3). Despite the deployment of resistant cultivars, serious losses can still occur. In 2007, for example, there was a severe epidemic of leaf rust in Kansas that caused an estimated 13.9% loss (1), the highest loss from a single wheat disease since estimates were begun in 1976. Contributing factors to this epidemic were virulence shifts that occurred in the leaf rust pathogen that rendered resistance in popular cultivars ineffective. Therefore, the duration of durability is of major importance with regard to the impact that host resistance will have on disease losses. Resistance durability is defined here as the length of time that a deployed resistant cultivar remains resistant to a particular disease; therefore, durability can be of relatively short, medium, or long duration. The goal of this review was to document the durability of resistance in deployed Kansas wheat cultivars to important diseases. The reaction of cultivars that were resistant to a particular disease when they were released was tracked over time to draw inferences on how long the resistance had lasted. If there was 2 August 2011 Plant Health Progress a substantial increase in the disease rating of a previously resistant cultivar, it was assumed that a new race of the pathogen had become prevalent resulting in susceptibility of the cultivar. On the other hand, if the disease rating remained low for many years, it was assumed that the pathogen had not developed a new race with increased virulence toward that cultivar. The duration of durability of resistance was determined for leaf rust, stem rust, stripe rust, tan spot, Septoria tritici blotch, wheat soilborne mosaic, wheat spindle streak mosaic, and wheat streak mosaic. These diseases are among the most important that occur in Kansas and there is a long enough history of resistance being deployed against them to allow durability data to be collected. Tracking Effectiveness of Resistance Beginning in 1983, Kansas State University has published disease ratings for about 60 different wheat cultivars each year except for 1989 and 2005. Data for disease reactions were obtained by extension personnel as they toured breeding nurseries, producer fields, and replicated cultivar performance trials where there was sufficient disease to obtain accurate assessments of cultivar reaction to pathogens. Data from numerous disease phenotyping experiments (5,6) were also used in the assignment of scores. Notes on disease reaction were synthesized by the extension specialist into a score that was reported in the annual version of the publication [e.g., see (9)]. From 1983 through 1990, disease ratings were presented using a scale of R (resistant), MR (moderately resistant), I (intermediate), MS (moderately susceptible), and S (susceptible). Beginning in 1991, disease ratings were presented using a 1-to-9 scale where a value of 1 indicated the cultivar was highly resistant and a value of 9 indicated high susceptibility. To allow comparisons between the two rating scales, ratings of R were considered equal to 1, MR equaled 3, I equaled 5, MS equaled 7, and S equaled 9. Data from the publications mentioned above were used to track disease ratings over time for individual wheat cultivars to specific diseases. Certain commercial cultivars were selected for presentation here because of their popularity and to show common trends. Coupled with these scores were data on the percentage hectares in Kansas that was planted to various cultivars (20). Using these data, charts were produced for cultivars of historical importance to Kansas: Arkan (Fig. 1), Karl (and a selection from Karl named Karl 92) (Fig. 2), and Jagger (Fig. 3). Disease reactions for these cultivars illustrate trends for durability of resistance to the foliar diseases leaf rust (caused by Puccinia triticina), Septoria tritici blotch (caused by Mycosphaerella graminicola), and tan spot (caused by Pyrenophora tritici-repentis). Data on reactions to stem rust (caused by Puccinia graminis f. sp. tritici) and stripe rust (caused by Puccinia striiformis f. sp. tritici) were collected but are not presented. Additionally, data for the reaction of cultivars to three virus diseases (wheat soilborne mosaic, wheat spindle streak mosaic, and wheat streak mosaic) are summarized in Table 1 and Table 2. Durability of resistance in a cultivar to a particular disease was considered of short duration if the disease rating rapidly went from low to high soon after the release of the cultivar. Durability was considered of moderate duration if the rating for a cultivar was low for a number of years after its release and then rose, or if the rise was gradual over several years. Durability was considered of long duration if the resistance rating was consistently low over many years or only showed an inconsequential increase. It should be noted that the amount of disease pressure during the time a cultivar was popular (Table 3) could have affected the duration of durability as defined here. However, the reason that a cultivar showed resistance durability of long duration is a separate issue and not necessarily known. 2 August 2011 Plant Health Progress Fig. 1. Reaction of wheat cultivar Arkan to leaf rust and percentage Kansas hectarage planted to Arkan. Disease ratings were on a 1-to-9 scale where 1 is highly resistant and 9 is highly susceptible. 2 August 2011 Plant Health Progress Fig. 2. Reaction of wheat cultivars Karl and Karl 92 to three foliar diseases and percentage Kansas hectarage planted to Karl and Karl 92. Disease ratings were on a 1-to-9 scale where 1 is highly resistant and 9 is highly susceptible. 2 August 2011 Plant Health Progress Fig. 3. Reaction of wheat cultivar Jagger to three foliar diseases and percentage Kansas hectarage planted to Jagger. Disease ratings were on a 1-to-9 scale where 1 is highly resistant and 9 is highly susceptible. 2 August 2011 Plant Health Progress Table 1. Reactionw over time of selectedx Kansas wheat cultivars to wheat soilborne mosaic and wheat spindle streak mosaic and percentage hectares planted to the cultivars. w Reaction to diseases presented using a 1-to-9 scale where 1 is highly resistant and 9 is highly susceptible; percentage wheat area in Kansas planted to the cultivar is in parentheses. x Cultivars were selected to show the range of changes in reactions that were typically observed. y Ratings were not published in 1989 and 2005. z Cultivar area data were not collected in 1987. Yeary Disease and cultivar Soilborne mosaic Spindle streak mosaic Arkan Jagger Karl/Karl92 Newton Jagger Karl/Karl92 1983 1 (<0.2) − − 1 (38.5) − − 1984 1 (0.9) − − 1 (30.9) − − 1985 1 (6.3) − − 1 (25.7) − − 1986 1 (10.1) − − 1 (21.1) − − 1987 1 (-)z − − 1 (-)z − − 1988 1 (14.9) − − 1 (13.4) − − 1990 1 (6.8) − 1 (0.7) 1 (8.3) − − 1991 1 (3.2) − 1 (5.9) 1 (7.6) − 3 (5.9) 1992 1 (2.2) − 1 (11.5) 1 (5.8) − 3 (11.5) 1993 1 (0.8) − 1 (23.0) 1 (3.2) − 3 (23.0) 1994 1 (0.4) 1 (<0.2) 1 (23.6) 1 (2.5) 1 (<0.2) 3 (23.6) 1995 1 (<0.2) 1 (<0.2) 1 (22.4) 1 (1.6) 2 (<0.2) 3 (22.4) 1996 1 (<0.2) 1 (1.0) 1 (20.9) 1 (1.3) 2 (1.0) 3 (20.9) 1997 − 1 (6.4) 1 (22.1) 1 (0.6) 2 (6.4) 3 (22.1) 1998 − 1 (20.2) 1 (10.8) 1 (0.3) 2 (20.2) 3 (10.8) 1999 − 1 (29.2) 1 (5.9) 1 (0.4) 2 (29.2) 3 (5.9) 2000 − 1 (34.0) 1 (3.5) 1 (<0.2) 2 (34.0) 3 (3.5) 2001 − 1 (35.8) 1 (3.3) 1 (0.2) 2 (35.8) 3 (3.3) 2002 − 1 (42.8) 1 (3.6) − 2 (42.8) 3 (3.6) 2003 − 1 (45.2) 1 (3.2) − 2 (45.2) 3 (3.2) 2004 − 1 (40.9) 1 (2.3) − 2 (40.9) 3 (2.3) 2006 − 2 (19.7) 1 (1.1) − 4 (19.7) 3 (1.1) 2007 − 2 (17.1) 1 (1.0) − 4 (17.1) 3 (1.0) 2008 − 2 (14.7) 1 (0.8) − 4 (14.7) 3 (0.8) 2009 − 2 (8.5) 1 (0.8) − 4 (8.5) 3 (0.8) 2 August 2011 Plant Health Progress Table 2. Reactionx over time of selectedy Kansas wheat cultivars to wheat streak mosaic and percentage hectares planted to the cultivars. x Reaction to wheat streak mosaic presented using a 1-to-9 scale where 1 is highly resistant and 9 is highly susceptible; percentage wheat area in Kansas planted to the cultivar is in parentheses. y Cultivars were selected to show the range of changes in reactions that were typically observed. z Ratings were not published in 2005. Yearz Cultivar 2137 2163 Jagger RonL TAM 107 Tr