GrainGenes

MINNESOTA

CEREAL RUST LABORATORY, USDA-ARS

University of Minnesota, 1551 Lindig, St. Paul, MN 55108, USA.

D.L. Long, K.J. Leonard, D.V. McVey, M.E. Hughes, D.H. Casper, and J.J. Roberts.

The rusts of wheat in the United States in 1995.

Stem rust (Puccinia graminis f. sp. tritici). In 1995, wheat stem rust was found scattered in plots and fields throughout the southern plains and the lower Mississippi Valley wheat-growing area. In all of these areas, the wheat crop matured before severe rust built up, and losses to wheat stem rust were minimal.

Overwintering stem rust sites were found on susceptible wheat cultivars in southern Texas, Louisiana, southwestern Georgia, northeastern Arkansas, and southwestern South Carolina. During the last week in March, traces of stem rust were found in winter wheat fields southwest of Houston, southeast of San Antonio, and in a nursery at Beeville, Texas. Stem rust development in southern Texas and Louisiana was normal in 1995, but less than in 1993, the last year with significant wheat stem rust in the U.S. In 1993 and other recent years with greater than average stem rust, generally, late winter weather was warmer and early spring rainfall was greater than occurred in 1995. In mid-April, widespread stem rust (5 % severities) was found in commercial fields of CK9835 and Savannah in central Louisiana. These fields were sprayed with Bayleton to stop further stem rust development. By late April, traces of stem rust were found in winter wheat plots from central Texas to southwestern South Carolina. In early May, wheat stem rust was severe on susceptible cultivars in southern and central Texas nurseries. During the second week in May, stem rust severities were light around an overwintering site in a field of CK9835 in northeast Arkansas. In mid-May, stem rust was light on susceptible cultivars growing in plots in northeastern South Carolina. The wheat stem rust that infected plots in a southwestern South Carolina nursery probably overwintered in a field of volunteer CK9835 growing in close proximity to the plots. During the third week in May, stem rust was severe in a plot of Saluda in northwestern Arkansas and in a plot of Chisholm in southwestern Oklahoma. In late May, wheat stem rust was found in a wheat plot in southwest Missouri.

During the second week in June, wheat stem rust foci were observed in fields of soft red winter wheat in southern Illinois and in wheat plots in southwestern Indiana at the soft dough stage. In the center of the foci, 40 % severities were common; however, 8 feet from the center, only traces of rust were present. Stem rust spores apparently were deposited 4-6 weeks earlier, but expansion of the foci from sites of initial infections was restricted by frequent rains that kept the wheat canopy wet and prevented newly produced spores from being released and carried out of the canopy by wind currents. In mid-June, traces of wheat stem rust were reported in a west central Indiana nursery. In the northern soft red winter wheat area, losses were negligible (Table 2).

During late June, 10 % stem rust severities were found in northeastern and northwestern Kansas plots of `McNair 701' at the hard dough stage. The hot, dry weather in late May and early June in Kansas was not conducive to rust increase.

On June 19th, traces of wheat stem rust were found on the susceptible spring wheat cultivar Baart in southern Minnesota plots. In early July, traces of stem rust were found in a hard red winter wheat plot in southeastern North Dakota. In mid-July, traces of wheat stem rust were found in plots of the susceptible spring wheat Baart in east central South Dakota, southeastern North Dakota and west central Minnesota.

Table 1. Races of Puccinia graminis f.sp. tritici identified from wheat in 1995.

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Number Percentage of isolates of Pgt-race*

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State Source collected isolates QCCJ QCCS QFCS QCMS RCRS TPMK

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AR Field 1 3 100

Nursery 2 6 50 50

IL Field 2 6 100

Nursery 1 3 100

IN Nursery 3 9 100

KS Nursery 2 6 17 50 17 17

LA Nursery 4 11 100

MN Nursery 13 39 38 51 10

MT Field 1 3 100

Nursery 2 6 33 67

NC Nursery 2 6 100

ND Nursery 13 39 64 23 8 5

NE Nursery 1 3 100

OK Nursery 1 1 100

OR Nursery 4 10 40 60

SC Nursery 1 3 100

SD Nursery 1 3 100

TX Field 3 9 67 33

Nursery 7 17 18 18 41 24

VA Nursery 2 6 100

WA Nursery 3 9 11 89

USA Field 7 21 29 71

Nursery 62 177 31 6 19 4 4 37

Total 69 198 27 6 17 4 7 40

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*Pgt-race code, after Roelfs and Martens, Phytopath 78:526-533. Set four consists of Sr9a, 9d, 10, and Tmp.

Several factors delayed stem rust development in the central Great Plains. First, stem rust overwintering north of Texas was less than normal. Then, cool dry conditions in late winter in the southern plains and hot dry weather in late May and early June in the central plains were not conducive for rust increase. In spite of this, sufficient quantities of stem rust spores spread from the central Great Plains to initiate stem rust foci in susceptible spring wheat plots in the northern Great Plains. Stem rust from these foci developed normally. Commercial spring wheats were not damaged, because their stem rust resistance is still highly effective.

In mid-July. traces of wheat stem rust were detected in wheat plots in eastern Washington, but none was reported in fields.

Six Pgt-races were identified from 69 wheat collections made in the U.S. in 1995 (Table 1). Race Pgt-TPMK was the predominant race identified this year, as it was from 1974-89 and 1992-94. This race comprised 40 % of the isolates in 1995, compared to 41 % in 1994, 74 % in 1993, 53 % in 1992, and 36 % in 1991. Pgt-QFCS comprised 17 % of the isolates in 1995, 41 % in 1994, 16 % in 1993, 21 % in 1992 and 14 % in 1991. Pgt-QCCJ, the barley attacking race, comprised 27 % of the isolates in 1995, 26 % in 1994, 8 % in 1993, 21 % in 1992 and 14 % in 1991.

Wheat leaf rust (Puccinia recondita f. sp. tritici). During the last week in March, leaf rust was widespread in winter and spring wheat fields throughout southern and central Texas. Rust severities on lower leaves ranged from 0 to 20 % in fields and from 0 to 60 % in nurseries, which was normal for this date (Fig. 1). Generally, leaf rust overwinters throughout southern Texas. In late March, in northern Texas fields and nurseries leaf rust was more severe than 1994. Lack of moisture in northern Texas delayed further leaf rust development, but by mid-April the rains had returned. In late April, leaf rust severities in southwestern Oklahoma wheat fields and plots ranged from traces to 20 %. These represent normal rust severity readings for this area for late April. By mid-May, leaf rust severities in north central Oklahoma wheat fields and plots ranged from traces to 40 %.

In contrast to the 1993-94 winter, when little leaf rust overwintered in Kansas, normal amounts of rust overwintered in eastern and central Kansas in 1994-95, although overwintering was greater than normal in western Kansas. In some fields, scattered volunteer plants were found with significantly higher incidences of rust than the majority of the field. Because volunteer winter wheat plants normally emerge before the crop is planted, they tend to be rusted more heavily in the fall. Usually, the infections on volunteer plants do not survive the winter in Kansas. In mid-April, leaf rust was generally light in Kansas (Fig. 1), although a few locations were observed with heavier infections. Freezing temperatures in early April killed some infected leaf tissue in western Kansas, which delayed local rust build up. In late April, a hard freeze in western Kansas and northwestern Oklahoma killed much of the leaf tissue where rust had developed. By the second week in May, leaf rust was increasing at a very slow rate throughout Kansas, because the cool moist weather was not conducive for rust increase. The moist conditions kept the wheat canopy wet and created conditions whereby the spores remained within the canopy. Leaf rust was severe on the lower leaves of susceptible cultivars, e.g., `Karl 92', but rust development on the flag leaves was light. During early June in central Kansas, rust severities ranged from greater than 60 % to less than 10 % in some fields a short distance apart. In mid-June, hot dry weather in northern Kansas and southern Nebraska prematurely dried many wheat leaves, thereby slowing leaf rust development. In mid-June, in a wheat field in Rooks Co., Kansas, 80 % severities were observed on goatgrass (Ae. cylindrica) plants. The estimated loss from leaf rust in Kansas in 1995 was 5 % (Table 2), which is significantly more than the 1 % loss in 1994 but half the loss of 1993. During the third week in June, 40-60 % severities were found on susceptible wheat cultivars in east central Nebraska plots. In late June, in south central Nebraska fields, the hot dry weather prematurely dried many leaves on wheat plants, but on the few remaining green flag leaves, 40 % leaf rust severities were common. In this area, losses varied with local conditions, but some fields suffered losses in yield of greater than 5 %. On April 28, the first pustules of leaf rust were observed in the northern Great Plains on green winter wheat leaves just breaking winter dormancy at the Rosemount nursery in east central Minnesota. Leaf rust was found on the winter wheat cultivar Roughrider in southeastern North Dakota fields on June 2, and on June 15, the incidence and severity (traces) were relatively unchanged. During the third week in June, in southern Minnesota plots, the susceptible cultivar Baart had 10 % leaf rust severities on the lower leaves. During early July, 20-60 % severities were observed in southeastern North Dakota winter wheat plots and fields at soft dough stage. Only traces were observed on lower leaves in spring wheat plots and fields at early milk growth stage. In mid-July, severities of trace to 60 % were common on flag leaves in winter wheat plots in east central Minnesota and eastern South Dakota. Losses in North Dakota and South Dakota winter wheat fields were 1 %. In susceptible spring wheat plots, trace to 20 % severities were common. Because of resistance only traces of leaf rust developed in commercial fields, and, therefore, losses were minimal in spring wheats (Table 3). Only traces of rust were reported on durum wheat.

In the southeast U.S., during late March, leaf rust severities were generally light to moderate on susceptible southern soft red winter wheat in plots and fields (Fig. 1). The winter rainfall in these areas was above normal, creating favorable conditions for rust infection. Cool temperatures in January and February slowed rust development, but warm temperatures and moist conditions in March were favorable for rust buildup. By late April, light amounts of leaf rust were found on susceptible cultivars in nurseries and fields in eastern North Carolina. By early May, leaf rust severities were generally light to moderate on susceptible southern soft red winter wheat in plots and fields from northern Mississippi to southern South Carolina (Fig. 1). By the second week in May, leaf rust severities ranged from traces to 80 % in wheat plots and fields at the soft dough stage in southeastern North Carolina. These rust-infected plants provided leaf rust inoculum for wheats farther north. In mid-June, the hot dry weather dried the leaves prematurely and thereby restricted rust development. This year throughout the southern soft red winter wheat, an increase of rust occurred on Northrup King/CK9835, which was grown on significant acreage. In the southeastern states, losses ranged from 2-5 % (Table 2).

In mid-June, trace to 40 % leaf rust severities were observed in southern Illinois and southwestern Indiana in soft red winter wheat fields and nurseries at the soft dough stage (Fig. 1). Farther north in the northern soft red winter wheat areas, like northwestern Ohio, only traces of wheat leaf rust were found in fields and plots at the half-berry stage. However, in central Michigan, leaf rust was moderately severe by the second week in June, suggesting that leaf rust overwintered there. In mid-June, leaf rust was present in light amounts in winter wheat fields in eastern Wisconsin. During mid-June, traces of leaf rust were found in the lower canopy of winter wheat near Aurora in the Finger Lakes area of New York at the watery-ripe growth stage, suggesting that leaf rust may have overwintered there. In Illinois, Michigan, and Indiana, losses ranged from 2 to 4 %.(Table 2)

In late April, leaf rust was light in plots and fields in the Sacramento Valley of California. By the first week in May, in the San Joaquin Valley of California, wheat leaf rust was widespread and severe on susceptible cultivars in fields and nurseries. During the first week in May, leaf rust was light in eastern Oregon and eastern Washington fields. By the third week in June, leaf rust severities as high as 80 % were found in irrigated winter wheat at early dough stage in central Washington and severities were light to moderate in dryland wheat in central Washington and trace to light in the Palouse area of eastern Washington. In mid-July, in the Palouse area of Washington, 80 % severities were observed on susceptible winter wheat cultivars growing in nurseries whereas rust was less in fields. Winter wheat losses ranged from 5 % in California to 1 % in Washington.

In 1995, when compared to the previous 10 years or race surveys, 14 new races were identified: CBT-18; CCM-18; CCR-10,18; CGB-10; FCD-10; LCG-10,18; MBB-18; MBR-10,18; MCG-10,18; MFL-10; MJB-10; PBJ-10,18; PNR-10,18 and TDR-10 (Tables 4 & 5). In 1995, Lr16 virulence was identified in three different races (virulence combinations), CGB-10, MGB-10, and MGG-10, whereas in 1994 the only race with Lr16 virulence was MGD-10. Also in 1995, Lr17 virulence was identified in three different races, FCD-10, MBJ-10, and MCD-10 whereas in 1994 it was identified only in race MCD-10. The MBJ-10 and MCD-10 races were identified from collections made from the cultivar `Jagger', which is grown widely in Kansas. Both of these virulences individually comprised 1.1 % of the total US population. Race MBR-10 was the most predominant race identified this year, as it also was in 1994, and comprised 27.8 % of the isolates in 1995 compared to 36 % in 1994 and 6 % in 1993. Race MBG-10, the most frequently identified race from 1989-93, comprised 20.8 % of the isolates in 1995, 16 % in 1994 and 20 % in 1993. Forty-nine Prt- races comprised the other 51.2 % of the U.S. population. Race SBD (virulent on Lr1, 2a, 2c, 17) was identified from two Ae. cylindrica collections made in southwestern Oklahoma. This is the same race identified from Ae. cylindrica collections in previous surveys. The interesting thing about this race is that it is avirulent on Lr3 and Lr10 and that it has not been identified from collections made from wheat.

Insert Figure 1, page 283.

Insert Table 2. page 284.

Insert Table 3. page 285.

Insert Table 4. page 286.

Insert Table 5, page 287.

Wheat stripe rust (Puccinia striiformis). The first report of wheat stripe rust in the central U.S. was in early April in southeastern Arkansas. By mid-April, stripe rust was found in fields in central Arkansas and by the last week in April, traces of stripe rust were found in plots of southern soft red winter wheat cultivars in north central Texas. In mid-May, stripe rust was found in a field of `CK9835' in northwestern Arkansas and in a plot of soft red winter wheat in south central Kansas. During late June, wheat stripe rust was light in Bozeman, Montana plots. In early July, traces of wheat stripe rust were found in an irrigated winter wheat field in the panhandle of Nebraska. Stripe rust generally is inhibited at temperatures above 70_F, so in these areas, the disease stopped developing with the onset of hot weather, and losses were minimal (Table 2).

During mid-April, stripe rust was observed in northwestern Washington wheat fields and nurseries. In late April, 70 % stripe rust severities were reported in some northwestern Washington wheat fields and nurseries, but losses were not significant in commercial fields with adult plant resistance. During the first week in May, light amounts of stripe rust were found in eastern Washington and eastern Oregon fields. Dry weather in late May retarded stripe rust development until late June. In early July, stripe rust was severe in plots of susceptible winter wheat in eastern Washington, but again stripe rust was not a problem in commercial fields with adult plant resistance.

In California, in late April, wheat stripe rust was light in San Joaquin Valley fields and Sacramento Valley plots. Stripe rust developed late and was less severe than normal for this area.