- 1995 -

Oat stem rust. Oat stem rust was observed in varietal plots in southern Louisiana in late February. In general, there was less oat stem rust and development was slower in this area than in 1994 because of the cool conditions in February and early March. Even though the rust developed later than normal, the rust still destroyed some of the southern Louisiana oat plots. During the last week in June, oat stem rust severities of 2% were observed in a northeastern Indiana nursery.

During the last week in March, oat stem rust overwintering sites (trace to 20% severities) were found in varietal plots in nurseries at Beeville and Temple, Texas and in a field 50 miles southwest of San Antonio. By the last week of April, trace to 20% severities were reported in oat plots in central and north central Texas. Traces of oat stem rust were also found in central Texas fields and on wild oat (Avena fatua) plants growing along the roadside. Oat stem rust was severe in north central Texas plots by the first week in May,. The amount of rust in this area was greater than in 1994, but within the normal range for stem rust development on oat in Texas. The relative lack of oat acreage in the central Great Plains (Oklahoma and Kansas) tends to interrupt potential epidemics of oat stem rust farther north unless stem rust is extremely severe in southern Texas.

The first detection of oat stem rust in the northern central plains in 1995 was during the first week in July, when traces of oat stem rust were found on wild oat plants growing alongside the road in southeastern North Dakota. In mid-July, traces of oat stem rust were found in plots in east central Minnesota, but no rust was found on a disease survey throughout eastern South and North Dakota. Much less oat stem rust was found in the northern Great Plains 1995 than in previous years. The reduced amount of oat stem rust seems to be associated with a decline in oat production. Oat acreage in the United States was the lowest since record keeping began in 1866. Losses to oat stem rust were minimal in the northern oat-growing area in 1995 (Table 1).

Race NA-27, virulent to Pg-1, -2, -3, -4 and -8, remains the predominant race in the U.S. population (Table 4). Race NA-5, virulent to Pg-3, and -15 continues to be the predominant race identified in California. Race NA-16, virulent to Pg-1, -3, and -8, was identified from collections made in Louisiana (nursery), Minnesota (nursery), North Dakota (A. fatua), and Texas (field, nursery and A. fatua).

Oat crown rust. During the last week in March, severe crown rust was observed in many southern Texas nurseries and fields. In some fields south of San Antonio, severities of 40% were common, while in late maturing cultivars in southern Texas nurseries, the rust was so severe it stunted the plants. The severe and widespread crown rust was comparable to the severe rust development in the southern U.S. in 1994.

In late March, crown rust levels were much higher than normal in the southeastern U.S. oat-growing area. In varietal plots crown rust was severe (>40%), while in oat fields, severities were moderate (1-20%). By mid-April, crown rust severities had increased in plots (>80%), while in fields severities were moderate (1-20%). Crown rust killed the most susceptible cultivars in this area by late April. By the first week in May, crown rust severities ranged from traces to 90% in oat plots at soft dough from east central North Carolina to central Louisiana. This was the most severe crown rust ever observed in this large an area in the Southeast. In northern Alabama, southern Tennessee, and northern Arkansas, however, crown rust infection was light in mid-May. The warm temperatures and moisture in the southeast were conducive for rust development. These southern areas of infection provided inoculum for the limited acreage of northern oat that had emerged by mid-May.

For the first time in the past 12 years crown rust was observed in the oat nursery at Wooster, Ohio. By mid-June, severities ranged from traces to 90% on the flag leaf. Also, for the first time in 35 years, crown rust was severe in the oat nursery near University Park in central Pennsylvania.

Pycnia were found on buckthorn, the alternate host, in the St. Paul, Minnesota nursery on May 17. The first infections on oat growing near the buckthorn nursery at St. Paul were found on June 5. During the third week in June, trace to 10% crown rust severities were found on lower leaves in southern Minnesota oat plots. During early July, trace to 60% crown rust severities were found on the flag leaves in oat plots in east central Minnesota. In early July, crown rust was light in southern Wisconsin but severe in eastern South Dakota. In mid-July, trace-20% crown rust severities were found on oat flag leaves throughout the eastern Dakotas, southern Minnesota and southern Wisconsin fields. The rust was most severe in fields where rust occurred early and conditions were conducive for rust development. Buckthorn growing in close proximity to oat fields provided some of the initial inoculum in these areas, i.e., southern Wisconsin and southern Minnesota. In mid-July, only traces of crown rust were recorded in northwestern North Dakota plots. By early August, 20% severities were observed on flag leaves in northwestern North Dakota plots and in an irrigated nursery in northeastern Montana. Severities of 60% were common on wild oat in northwestern Minnesota. Crown rust losses were severe in late planted oat in Minnesota and the Dakotas (Table 1).

During the second week in May, crown rust was light in oat fields at late milk stage in the Central Valley of California. During the third week in May, crown rust severities of 60% were in observed in Davis, California oat plots.

In early June, heavy aecial infections were observed on buckthorn bushes in the area around Saskatoon, Canada. However, only traces of crown rust were observed in plots by August 8. In mid-June, oat spreader plots in a buckthorn nursery in southern Ontario were heavily infected with crown rust.

Frequencies of virulence on oat lines with known Pc genes for resistance to crown rust are shown in Table 7 for isolates collected from different regions of North America and from Uruguay in 1995. In general, frequencies of virulence in the upper Midwest were more similar to those in Texas than in the Southeast. Virulence to Pc38 was more frequent in the Midwest than in Texas or the Southeast, but virulence to Pc50, Pc51, and Pc61 was more frequent in Texas and the Southeast than in the Midwest. Virulence to Pc48, Pc52, Pc53, Pc62, or Pc68 was rare in 1995. One isolate virulent to Pc68 was found in California. Collections from California and Mexico had higher virulence to Pc45 and lower virulence to Pc14 than collections from other parts of North America. Virulence to each of the oat crown rust differential lines was found among the isolates tested from Uruguay.

The oat crown rust population was highly diverse in all regions of North America. Based on a set of 16 oat crown rust differentials that includes Pc40, 45, 46, 50, 38, 39, 48, 68, 51, 52, 58, 59, 54, 56, 62, and 64, we found 50 races from Texas, 58 from the Southeast, and 83 from fields and nurseries in MN, ND, and SD. The most frequently found races in 1995 were PBLB (13 isolates) and PBLG (12 isolates) from Texas, LBBM (16 isolates) from the Southeast, and LQLB (4 isolates) and NBLG (4 isolates) from the Midwest. LB-- combinations were common in all parts of North America except California and Mexico, where JB-- and QL-- combinations were more common.
 

- 1996 -

Oat stem rust. In mid-April, traces of oat stem rust were found in a nursery in southeastern Louisiana. Usually, by early April oat stem rust is severe in these plots, but the cooler than normal weather in March slowed the disease progress. By early May, 20-30% oat stem rust severities were observed on susceptible cultivars in central Louisiana, which was less than normal. In mid-May, trace to 80% rust severities were observed on oat cultivars in southern Louisiana plots.

In late April, traces of oat stem rust were found in nurseries in central Texas. During the last week in June, the first detection of oat stem rust in the central plains was traces of oat stem rust in north central Kansas fields. By mid-July, traces of oat stem rust were found in plots in south central Minnesota and southeastern South Dakota and in one field in southeastern South Dakota as well as on wild oat in southeastern North Dakota. In early August, traces of oat stem rust were found in plots in northwestern Minnesota and northeastern North Dakota. Much less oat stem rust was found in the northern Great Plains in 1995 and 1996 than in previous years. The reduced amount of oat stem rust seems to be associated with a decline in oat production.

In early May, rust severities of 30% were found on oat growing in plots and traces on wild oat in the Sacramento Valley in California.

Race NA-27, virulent to Pg-1, -2, -3, -4 and -8, once again was the predominant race in the U.S. population (Table 5). Race NA-10, virulent to Pg-2, -3, and -15, was identified from two collections made in California. Race NA-16, virulent to Pg-1, -3, and -8, was identified from nursery collections made in Louisiana and Texas.

Oat crown rust. During the last week in March, traces of crown rust were found in plots in southern Texas, but none was found in commercial fields. During mid-April, crown rust was light in southern Texas fields and plots. In late April, crown rust was light in central Texas fields and plots. This area provided very little crown rust inoculum for oat-growing areas farther north.

By the second week in April, oat crown rust was found in a southeastern Louisiana nursery and 15% severities were reported in spreader rows. By early May, the severity of oat crown rust in central Louisiana was much less than normal.

During mid-May, light to moderate numbers of aecial infections were found on buckthorns growing in south central Wisconsin. Buckthorns are the alternate hosts for crown rust and generally provide the initial spores for crown rust infection of the northern oat crop. During the third week in May, light aecial infections were found in the St. Paul, Minnesota buckthorn nursery. By early June, light to moderate numbers of aecial infections were found on buckthorns growing in south central Minnesota and east central North Dakota. During the second week in June, light to moderate aecial infections were found on buckthorn in east central Illinois, southern Minnesota and east central South Dakota. Crown rust was beginning to build up on susceptible oat lines near buckthorn at St. Paul, Minnesota, by early June. During the third week in June, traces of crown rust were found on oat in fields in south central Wisconsin and Iowa and in plots in south central Minnesota and east central South Dakota. During the last week in June, a new wave of aecia were found on buckthorn growing in hedges in St. Paul, Minnesota. This is the latest that actively sporulating aecia were found on buckthorn in Minnesota in the last 8 years. Oat growing near these hedges were heavily infected with crown rust. By early July, crown rust was light to moderate in southern Wisconsin and southern Minnesota oat fields. During the second week in July, traces of crown rust were found in an oat field in west central Minnesota and in the same field, 20% severities were found on wild oat plants. In mid-July, trace-40% crown rust severities were found on oat flag leaves in plots and fields throughout the Dakotas and southern Minnesota. In plots in southeastern South Dakota, 40% severities were common, while in central North Dakota plots, trace severities were common. On wild oat, severities of 20-40% were common in southeastern North Dakota fields. Crown rust was reported light to moderate in south central and southeastern Wisconsin. The most severe crown rust was found where rust occurred early and conditions were conducive for rust development. Buckthorn growing in close proximity to oat fields provided the initial inoculum in these areas, i.e., southeastern South Dakota and southern Minnesota. In early August, crown rust was severe in northern Wisconsin fields. Losses were more severe than in 1995 in the northern oat-growing area with the latest planted fields suffering the most damage (Table 2).

During mid-April, traces of crown rust were found on wild oat growing in the Sacramento Valley of California.

In mid-June, aecial development was severe in the Guelph, Canada area and losses occurred in the latest maturing oat fields.

In 1996, relatively few isolates of oat crown rust were obtained from Texas and the Southeast. Virulence to Pc38 and Pc39 was more frequent in the Midwest than in Texas or the Southeast, whereas virulence to Pc45, 50, 51, 57, 61, and 67 was higher in Texas and the Southeast than in the Midwest (Table 8). The high levels of virulence on Pc45 and Pc54 were unusual for Texas and the Southeast. Virulence on these differentials typically was high in collections from California and Mexico in past years. This suggests that there may have been an influx of oat crown rust into Texas from Mexico in 1996. Aecial collections from the St. Paul, MN buckthorn nursery had higher frequencies of virulence than uredinial collections from fields in MN, ND, and SD in 1996. This suggests that the genetic diversity of oat lines in the nursery and the strong influence of the sexual stage of the pathogen promotes higher levels of virulence. Virulence to Pc48 was relatively common in 1996 in contrast to earlier years. Virulence to Pc62 and Pc68 remained rare, although Pc68 virulence was found both in California and Mexico.

Fourteen races were found in Texas in 1996. PBLG (4 isolates) was the most common. From the Midwest uredinial collections, 51 races were found, with LBBG (5 isolates) the most common. Races NDQB and NDRG were most common in the Northeast, where 22 races were found. The population in Mexico also was diverse; 39 races were found with GBBB (5 isolates) the most common. Several of the races found in California were also found in Mexico. Two isolates of race JBBL were found in California.
 

- 1997 -

Oat stem rust. In mid-February, traces of oat stem rust were found in the nursery plots at Beeville in southern Texas. During the first week in April, hot spots of oat stem rust 0.5 m in diameter were found in southern Texas fields and nursery plots. Along the roadside in central Texas, traces of stem rust were observed on wild oat. The rains during the first week in April were good for rust infection and the rust increased significantly. During the second week in May, oat stem rust was increasing rapidly in varietal plots in College Station. The Texas oat stem rust provided inoculum for areas farther north, but the lack of oat acreage in the central Great Plains tended to interrupt potential epidemics.

In 1997, by late March, stem rust was increasing in oat varietal plots in southern Louisiana where it generally can be found every year by early March. During the second week in April, severities of 50% were observed in varietal trial plots in southwestern Alabama. During the last week in April, overwintering centers of stem rust were found in an oat field in east central Louisiana and in oat varietal plots in southern Georgia, southwestern Alabama, southwestern Mississippi and central Louisiana. Oat stem rust was severe in varietal plots and fields throughout central and northeastern Louisiana by mid-May.

In early May, oat stem rust was found in plots in the Sacramento Valley of California.

In mid-July, traces of oat stem rust were found in plots in east central South Dakota, central, and west central Minnesota. These were the first reports of oat stem rust in the U.S. since mid-May, when oat stem rust was found in fields and plots in Texas, Louisiana and Alabama. Much less oat stem rust has been found in the northern Great Plains the past three years than in previous years. The reduced amount of oat stem rust seems to be associated with a decline in oat production.

Race NA-27, virulent to Pg-1, -2, -3, -4 and -8 continued to be the predominant race in the U.S. population (Table 6). Race NA-5, virulent to Pg-3, and -15, was identified from a nursery collection made in Idaho. This race was not found in California in 1996 or 1997, where it had been the predominant oat stem rust race. Race NA-10, virulent to Pg-2, -3, and -15, was identified from nursery collections made in Idaho and California. Race NA-16, virulent to Pg-1, -3, and -8, was identified from collections made in Louisiana (field and nursery) and Mississippi (nursery).

Oat crown rust. By late February, crown rust was found in southern Texas plots and the rust was much more severe in 1997 than in 1996. During the first week in April, crown rust was severe in southern and central Texas fields and plots. Sixty-percent severities were common on the most susceptible cultivars in nursery plots and in some cultivars, the rust was killing the host. In some fields in southern Texas, overwintering hot spots 3-m in diameter with 40-60% severities were observed. In late April, crown rust was severe in central Texas varietal plots, while in oat fields, severities were moderate(1-20%). The Texas oat crown rust inoculum was available for areas farther north, but the lack of oat acreage in the central Great Plains tends to interrupt potential epidemics.

In late March, crown rust was severe in varietal plots in southern Louisiana. By mid-April, crown rust was severe (>80%) in oat varietal plots and moderate (1-20%) in oat fields. In plots in southwestern Alabama and central Louisiana, the rust was so severe it killed some of the oat. This widespread crown rust development is equal to the rust development of the last three years in the southeastern U.S. Crown rust development in the southeastern U.S. provided inoculum for areas farther north. In mid-June, traces of oat crown rust were found in oat fields in west central Indiana.

Crown rust pycnia appeared on buckthorns in St. Paul, Minnesota, on May 19. The infections apparently came from basidiospores released from germinating teliospores on infected straw during rains on May 7-8. In late May, aeciospores, which can infect oat, were found on buckthorn leaves in southern Minnesota and southern Wisconsin. In early June, moderate to severe aecial infections were found on buckthorn bushes in south central and southeastern Wisconsin. By June 17, crown rust severities of 20% were observed on lower leaves of oat growing near the buckthorn bushes in the nursery on the University of Minnesota, St. Paul campus. By the second week in June, traces of crown rust were found in southern Wisconsin oat fields. Crown rust developed slowly due to cool weather in the northern oat-growing area. By June 30, crown rust infection was severe (50-80%) on susceptible oat cultivars near the buckthorn nursery on the University of Minnesota, St. Paul campus. In early July, light levels of oat crown rust were detected in a few fields in southern Minnesota, central Wisconsin, and south central Pennsylvania. During mid-July, crown rust severities ranged from trace to 10% in oat fields and trace to 40% on flag leaves in plots in west central and southern Wisconsin. In late July, oat crown rust severities ranged from trace to 60% in plots and trace to 10% in fields in northwestern Minnesota and northeastern North Dakota. The most severe crown rust was found where rust occurred early and conditions were conducive for rust development. Buckthorn growing in close proximity to oat fields provided the initial inoculum in these areas, i.e., southern Minnesota and southern Wisconsin.

In early June, aecial development was light on buckthorn bushes in eastern South Dakota. In early July, crown rust severities of 10% were found on lower leaves of susceptible oat cultivars in eastern South Dakota varietal plots. During mid-July, crown rust severities ranged from trace to 5% in oat fields and trace to 40% on flag leaves in plots in eastern South Dakota and east central North Dakota. No crown rust was found in oat plots in central South Dakota. Lack of moisture in May and cooler than normal weather in early June delayed rust development, but warmer and rainy weather than normal in early July favored crown rust increase. Rainy weather, however, washed much of the inoculum off infected plants before spores could spread. This was one of the lowest levels of crown rust infections on buckthorn and oat in recent years in South Dakota.

In 1997, virulence on Pc46, 50, 51, 60, and 61 was more frequent in collections from Texas than from the Midwest (Table 9). For the most part, virulence frequencies among isolates from the Southeast were similar to those from Texas. Isolates from Mexico were unusual in their relatively low frequency of virulence on Pc45. Crown rust isolates from Mexico in 1997 tended to resemble those from the Midwest and from Texas and the Southeast. A notable exception was that four isolates from Mexico were virulent on oat lines with Amagalon resistance (data not shown). Virulence to Amagalon rarely has been found in the field in the U.S. As in 1996, aecial collections from the St. Paul, MN buckthorn nursery were generally more virulent than were the uredinial collections from fields in the Midwest. Virulence to Pc53, 62, and 68 was rare in North America in 1997. In 1997, 55 isolates of oat crown rust were obtained from Brazil. The Brazilian isolates had high virulence on most differentials except Pc38, 50, 63, and 68. Surprisingly, nearly all of the Brazilian isolates were virulent on TAM-O-393, which has effective resistance to all 1997 U.S. isolates except one from California and five from the St. Paul, MN buckthorn nursery (data not shown).

Twenty four races were found in Texas and 42 in the Southeast in 1997. The most common race in Texas was PBLG (5 isolates), and the most common race in the Southeast was LBBB (4 isolates). Uredinial collections from the Midwest contained 46 races; race LQBB was found 3 times, but no other race appeared more than twice. All eight isolates from California were separate races. Oat crown rust collections from Mexico contained 78 races, with LBBG (4 isolates) the most common. There were 45 races among the Brazilian isolates; race SJLM occurred five times among the 55 isolates.

Losses to crown rust were less severe than in 1996 in the northern oat-growing area, with the latest planted fields suffering the most damage (Table 3).

 ¹ See Martens et al., Phytopathology 69:293-294.