ITEMS FROM THE UNITED STATES

MINNESOTA

CEREAL DISEASE LABORATORY
(formerly CEREAL RUST LABORATORY)
USDA­ARS, University of Minnesota, 1551 Lindig, St. Paul, MN 55108, USA.

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

The rusts of wheat in the United States in 1997.

Stem rust. In 1997, wheat stem rust was found scattered in plots and fields throughout the lower Mississippi Valley wheat-growing area. Throughout the area, losses to wheat stem rust were light, but these fields did provide stem rust inoculum for susceptible wheats and barleys farther north.

During 1997, wheat stem rust overwintering sites were found in late April in central and east central Louisiana. For example, stem rust foci were scattered throughout a 40-acre field of CK 9835, although in varietal plots 60 miles away, the rust was so severe that much of the wheat in some plots was killed by stem rust. These sites in Louisiana were the only locations where wheat stem rust was found during rust surveys through the southeastern U.S., southern Oklahoma, and northern Texas in the last week of April. No stem rust overwintering sites were found in south Texas, and in early May only traces of stem rust were reported in central Texas varietal plots. Stem rust development in Texas this year was less than normal.

In late May, traces of wheat stem rust were found in plots in northeastern Louisiana and northwestern Arkansas. During the second week in June, wheat stem rust foci 1 meter in diameter were found in southeastern Illinois and northwestern Kentucky fields. The next report of wheat stem rust in the SRWW area was in early July, in a nursery in south central Virginia.

In mid-July, traces of wheat stem rust were found in east central South Dakota plots of highly susceptible spring wheat cultivars such as Morocco, and by the fourth week in July, 40 % severities were observed in other susceptible spring wheat lines. In mid-July, traces of stem rust were found in a plot of the susceptible spring wheat Max in east central North Dakota. The infections on Max were on the leaf sheaths and originated from spores that were rain deposited 7 and 14 days earlier. The stem rust infections in the northern plains this year may have originated from rust spores that were released from rusted SRWW fields in southern Illinois or the Ohio Valley area. No other potential sources of wheat stem rust were known to have existed at the time these infections occurred. In 1997, there were few reports of stem rust in fields and nurseries, and the number of stem rust collections received at the Cereal Disease Lab was much less than normal.

Several factors delayed stem rust development in the northern plains: first, little stem rust overwintered in the southern U.S.; second, dry weather in June limited infection; and third, stem rust resistance in the spring wheats remains highly effective in the northern Great Plains.

In mid-July, small foci of stem rust were found on winter wheat cultivars in the Palouse region of the Pacific Northwest. In late July, in late maturing cultivars like Eltan, stem rust was severe and caused yield losses. Stem rust was severe in a few of the susceptible spring wheat cultivars in eastern Washington and northern Idaho and caused reduced grain weights in some fields.

Six Pgt- races were identified from 42 wheat collections made in the U.S. in 1997 (Table 1). The small number of stem rust collections probably relates to the lack of rust overwintering in southern states. Race Pgt TPMK was again the predominant race identified as it was from 1974­1989 and 1992-1996. This race comprised 69, 65, 40, 41, and 74 % in 1997, 1996, 1995, 1994, and 1993, respectively. Race Pgt-QCCS, comprised 12 % of the isolates in 1997 and none in 1996. Pgt-QFCS comprised 7, 26, 17, 41, and 16 % of the isolates in 1997, 1996, 1995, 1994 and 1993, respectively. Race Pgt-RCRS comprised 9 % of the isolates in 1997, and only traces of the race were identified in the 1996 survey. Race Pgt-QTHJ comprised less than 2 % of the population this year. Race Pgt-QCCJ, the barley attacking race, was not identified in the 1997 survey. Pgt-QCCJ comprised 1, 27, 26, 8, 21, and 14 % of isolates identified in 1996, 1995, 1994, 1993, 1992 and 1991, respectively, and the barley cultivars from which this race was identified previousely are still being grown.

Wheat leaf rust. Southern Plains. In contrast to stem rust, more leaf rust overwintered in the southern Great Plains in 1997 than the year before. In late November, leaf rust was found in many locations in Oklahoma and Texas. In mid-March, wheat leaf rust was heavier than normal in plots and fields of susceptible soft and hard wheats throughout southern and central Texas. By late March, leaf rust was unusually heavy in Oklahoma, where it survived the winter (Fig. 1). During the first week in April, wheat leaf rust severities of 60 % were observed on lower leaves of susceptible cultivars in fields and plots throughout southern and central Texas. Cool, wet conditions in many parts of Texas and Oklahoma during mid-April created conditions conducive for rust increase. In mid-April, 80 % severities were reported on flag leaves of TAM 200, TAM 107, and 2163 at the central Texas nurseries. Leaf rust was more severe in the central Texas nurseries at McGregor and Temple than at the two southern nurseries at Beeville and Uvalde. In some fields of susceptible cultivars in central Texas, 40 % severities were observed on the upper leaves at the soft dough stage, and losses to leaf rust occurred in these fields. During the last week in April, wheat leaf rust severities in north central Texas and southern Oklahoma fields ranged from trace to 2 %, and severities in plots ranged from trace to 40 %. Most commercial cultivars in Texas expressed susceptible reactions to leaf rust this year, which suggested that new races were present in the area. The leaf rust race identifications (Table 2 and Table 3) confirmed that a change in the Texas race population has occurred this year. Twenty-four races were identified in Texas this year, including four (MDGL, MDRB, TDSL and THBL) not identified previously in Texas. At the same time, the leaf rust resistance in the cultivars Big Dawg, Longhorn, and Tomahawk held up fairly well. During the third week of May, in north central Oklahoma, a wheat leaf rust severity of 60 % was observed on the flag leaves of many of the cultivars growing in fields and plots. In north central Oklahoma varietal plots, cultivars Karl 92, 2163, and Chisholm had 60 % severity readings, whereas the severities in cultivars Custer, Jagger, and 2137 were less than 5 %. Estimated rust losses to leaf rust in Oklahoma this year were 10 % (Table 4).

In mid-May, leaf rust severities of 20 % were observed on Ae. cylindrica (goatgrass) in western Oklahoma. This was the heaviest leaf rust observed on goatgrass in the last 5 years.

Central Plains. In mid-March, overwintered leaf rust was found in the southern tier of Kansas counties. By early April, rust pustules were found on the lowest leaves in southern Kansas and farther north in Kansas, rust was found on the top two leaves. On 12-13 April in Kansas, frost damaged the head and stem but did not destroy all of the rust-infected leaves. Leaf development was set back somewhat by the freeze, but enough leaf rust survived to still cause significant losses in Kansas and provide inoculum for the wheat-growing areas farther north. In late April, leaf rust was found on flag leaves in extreme southern Kansas, but development was slower than expected because of the cool weather. Early in May, leaf rust was found on flag leaves of wheat in the southern tier counties of Kansas (excluding the far southwest). By mid-May, leaf rust severity on flag leaves in south central Kansas was as high as 40 % in some fields of susceptible winter wheat cultivars. In plots in the same area, severities on the flag leaves ranged from 0-60 %. Leaf rust also was found on flag leaves throughout central and northern Kansas, but with little rust on the lower leaves. The lack of rust on lower leaves indicates that the flag leaf infections came from spore showers from outside the region. The timing of the infection indicates that Texas was the most likely source of the spores. In mid-May, wheat in the west and south central areas of the state was under moisture stress. In late May, in south central Kansas fields, leaf rust severities of 80 % were common on the flag leaves of susceptible winter wheat cultivars where rust overwintered. In varietal plots in south central Kansas, leaf rust decimated most cultivars. The only cultivar showing some resistance was Big Dawg. Throughout northern Kansas, leaf rust development was slow. The cooler-than-normal night temperatures during the last part of May may have been partly responsible for the slow development. By the second week in June, leaf rust severities of 10 % were observed in wheat fields in north central Kansas at soft dough. Estimated leaf rust loss in Kansas was 3.7 % (Table 4).

In late May, traces of leaf rust were observed in plots and fields in southern Nebraska. By the second week in June, leaf rust severities of 10 % were observed in wheat fields in south central Nebraska (see Fig. 1). In early June, traces of leaf rust were observed on lower leaves of wheat in eastern Colorado. In late June, leaf rust severities ranged from trace to 80 % on cultivars in south and west central Nebraska varietal plots. In southern Nebraska fields of susceptible cultivars losses, to leaf rust averaged 1 % (Table 4).

Northern Plains. On 2 June, trace amounts of leaf rust were found in south central and southeastern North Dakota winter wheat plots. Initial leaf rust development in the northern plains came mainly from windborne spores from the south that were deposited with rains. During the second week in June, traces of leaf rust were found in southeastern South Dakota fields and plots and in winter wheat plots in east central Minnesota. Fields of susceptible winter wheat in central South Dakota had 50-80 % leaf rust severities on flag leaves during the first week in July (see Fig. 1). During the second week in July, 60 % severities were observed in fields of winter wheat at the mid-dough stage in southeastern North Dakota. Winter wheat losses to leaf rust in South Dakota averaged 4 % (Table 4). Light losses occurred on winter wheat in North Dakota. Leaf rust severities of 60 % were reported on flag leaves in spring wheat plots in west central Minnesota, east central South Dakota, and east central North Dakota in mid-July. Spring wheat fields in eastern South Dakota and North Dakota had only traces of leaf rust, because of their moderate to high levels of resistance to prevailing races. In late July, trace to 10 % severities were reported on spring wheat in northern North Dakota and northwestern Minnesota fields. Although most of the spring wheat cultivars in the northern plains were resistant to leaf rust, some cases of higher than usual leaf rust severities were reported. Losses were more severe than usual, and most of them were in late-planted fields (see Table 5). No rust was reported on durum wheat in the Great Plains.

Southeast - In the southeastern U.S., in early spring, wheat leaf rust was severe in both fields (see Fig. 1) and plots of susceptible SRWW cultivars. The leaf rust was much more severe than last year in early spring. The winter rainfall in the southeast was normal, creating favorable conditions for rust infection. In southern Louisiana in early April, many of the cultivars that previously were resistant were showing significant rust development. During the last week in April, 60 % severities on flag leaves were observed in plots and fields of susceptible SRWW cultivars in the southeastern U.S. In southern Arkansas, during early May, leaf rust development on flag leaves was severe in many late-planted fields of susceptible cultivars. Cultivars most affected were Wakefield, Hazen, Jackson, Coker 9803, Hickory, and Pioneer 2580. Although the leaf rust epidemic in southern Arkansas was too late to cause major damage to the entire wheat crop, this was the most significant rust development in several years and caused damage in late-maturing fields. In late May, wheat leaf rust was severe in varietal plots and light in fields in the boot heel of Missouri, northeastern Arkansas, and western Tennessee. During the first week in May in the coastal plain of South Carolina, wheat leaf rust pustules were found on flag leaves; by mid-May, wheat leaf rust caused premature senescence of the state's predominant cultivar, Coker 9835.

Midwest. During the second week in June, trace leaf rust ratings were recorded on most of the wheat cultivars in southern Indiana plots, although 20 % severities were recorded on a few susceptible lines. By early June, traces of leaf rust were reported in SRWW fields in southeastern Wisconsin, south central Michigan, and northwestern Indiana. Losses were light in this area, because rust developed later than normal.

California. During the third week in April, leaf rust was light in wheat fields in the San Joaquin Valley, and by late April, leaf rust was found throughout Sacramento Valley fields. Because wheat leaf rust developed so late, losses were light throughout the state.

Northwest. In late May, wheat leaf rust was light in the Skagit Valley of northwestern Washington. In early June, in eastern Washington, 1-2 % severities were reported in some fields, and leaf rust also was increasing in the Willamette Valley of western Oregon. In early July, wheat leaf rust was severe on susceptible cultivars growing in varietal plots in the Skagit Valley in northwestern Washington and the Palouse region of eastern Washington and northern Idaho. In commercial fields in these areas, leaf rust developed late and caused slight damage to the winter wheat crop, and most of the spring wheat cultivars had adequate adult plant resistance. In late July, wheat leaf rust was increasing in spring wheat fields in the P