Oat Rusts in the United States in 2001

D. L. Long, K. J. Leonard, M. E. Hughes, G. E. Ochocki and L. A. Wanschura

Cereal Disease Laboratory, USDA-ARS, University of Minnesota, St. Paul, MN 55108

davidl@cdl.umn.edu, kurtl@cdl.umn.edu, markh@cdl.umn.edu, jerryo@cdl.umn.edu and lucyw@cdl.umn.edu

Oat stem rust. During the third week in March, oat stem rust severities ranged from traces to 10% on the leaves of cultivars Chapman, Harrison and two experimental lines in the nursery at Beeville in southern Texas. In late March, a stem rust collection was made from wild oat (Avena fatua) in southern Texas. In early April, in a southern Texas field, oat stem rust was found and in mid-April, light stem rust was found in a central Texas field. In early May, light oat stem rust was found in central Texas plots. The cooler than normal temperatures in early spring slowed oat stem rust development throughout Texas.

In late April, in a field of the cultivar Chapman near Fairhope in southern Alabama, scattered centers of oat stem rust with 60% severities were found. The over-wintering centers were 2 meters in diameter. Rust development was slow in these centers because of the cool temperatures in March, while the April weather was warmer but moisture was limiting.

In late April, oat stem rust was severe in nursery plots in Baton Rouge, Louisiana where the crop was near maturity. In early May, light oat stem rust was found in north central Louisiana plots. Dry weather in April in the southern U.S. slowed oat stem rust development.

In mid- July, in southern Minnesota and east central South Dakota oat plots and fields, trace to 20% stem rust severities were observed. Stem rust developed late and therefore losses were minimal.

On May 25, limited oat stem rust was found on wild oats (Avena fatua) in Sonoma Co. California. In general, oat stem rust development was equal to last year throughout the southern U.S.

Race NA-29 (avir/vir formula: 9,13,16,a/1,2,3,4,8,15) was the predominant race identified in 2001 (Table 2) comprising 56% of the total isolates. In 2000, it comprised 64% of the total isolates. NA-27 (9,13,15,16,a/1,2,3,4,8), the predominant race in recent years up until 2000, was the second most commonly identified race comprising 20% of the isolates in 2001. Races NA-5 (1,2,4,8,9,13,16,a/3,15) and NA-10 (1,4,8,9,13,16,a/2,3,15) were identified from collections made in California. NA-67 (16,a/1,2,3,4,8,9,13,15) was identified from collections made in Texas, South Dakota, North Dakota and Minnesota.

Oat crown rust. In late March, in Beeville, Texas oat plots, crown rust was moderate to severe in susceptible cultivars. In the oat plots in the Uvalde, Texas nursery crown rust was light and in a field in the immediate area a few pustules of crown rust were found. In early April, oat crown rust was severe in some fields in southern Texas and light in a few central Texas fields. In mid-April, crown rust was severe on common oat and wild oat (Avena fatua) growing alongside roadsides in central Texas. In early May, crown rust was light in fields and moderate on susceptible cultivars in plots in central Texas. In late May, 10% crown rust severities were observed in an oat field in north central Oklahoma.

By the second week in April, crown rust was light in southern Louisiana varietal plots. The average cultivar was in the late boot stage, which was about 10 days later than normal. In late April, 20% severities at the early milk maturity stage were observed in southern Alabama oat nursery plots. By early May, crown rust was severe in Baton Rouge, Louisiana plots. In 2001, crown rust development in the southern U.S. was lighter than normal and these locations provided minimal rust inoculum for susceptible oat growing farther north.

In late June, 40% crown rust severities were observed in two oat fields in north central Kansas and south central Nebraska. During the first week in July, light crown rust was observed in oat fields in southern Wisconsin.

During the second week in May, aecial infections were observed on bushes at the St. Paul, Minnesota buckthorn nursery. The aecial development in the buckthorn nursery was 2 weeks behind last year, but near normal for this date. In late May, the plentiful moisture and warm temperatures were ideal for aecial infection. By early July, oat growing in spreader rows near the buckthorn nursery in St. Paul, Minnesota, had severe (60% severity) crown rust infection on the upper leaves. Traces of crown rust were found on oat in the other St. Paul nurseries. In early July, from trace to 40% crown rust severities were found in a east central North Dakota nursery. By mid-July, oat crown rust (trace to 60% severities) were observed in some east central South Dakota and west central Minnesota fields. Crown rust was found on cultivated oat in plots and wild oat throughout North Dakota and Minnesota in the last week of July. Severity levels were generally light to moderate on wild oat. Some oat cultivars in breeding plots in North Dakota had moderate to high levels of crown rust infections. In 2001, crown rust was lighter than normal throughout the northern oat growing area. Since crown rust was slow to develop in the northern oat growing area, losses were less than normal.

In mid-May, 50% rust severities were observed in plots in Davis, California and a collection of crown rust was made from Avena fatua in Sonoma County, California.

Relatively few isolates of crown rust were obtained from oat in 2001; 44 were collected from Texas, 33 from midwestern states (MN, ND, and SD), and only two each from Alabama and California. Frequencies of virulence to Pc36, Pc56, and Pc70 increased substantially in both Texas and the Midwest in recent years compared to mean values for 1991 through 2000, while frequencies of virulence to Pc50 declined in both areas (Table 1). Frequencies of virulence to Pc35 and Pc61 increased in Texas but not in the Midwest. Frequencies of virulence to Pc48 and Pc52 increased in the Midwest in 2001, presumably due to increased use of Pc48 there. Virulence to Pc52 appears to be associated with virulence to Pc48. Virulence to Pc59 and Pc71 also appeared to increase in the Midwest in recent years. In 2001, virulence to Pc38, Pc39, and Pc63 declined somewhat from virulence levels in 2000 in both Texas and the Midwest to levels more typical of mean values for the period 1991 through 2000. In the 1980s before widespread use of Pc-38 and Pc-39 in the Midwest, virulence to these genes was rare in both Texas and the Midwest. Virulence to Pc63 is associated with virulence to Pc38. The apparent decline in virulence to Pc46 in 2001 is problematic, because off-type seed accidentally became mixed with the Cereal Disease Lab source of the Pc46 line sometime during 2001, but the contamination was not detected until after tests on the 2001 survey collections had been completed. Thus, a portion of the isolates recorded as avirulent to Pc46 may actually have been virulent. This is more likely for isolates from the Midwest than other regions, because the Midwest isolates were tested last.

Oat crown rust races were identified according to the nomenclature of Chong et al, 2000. There were 29 races identified among the 44 Texas isolates and 27 races among the 33 midwestern isolates from 2001. From the total of 81 isolates from the U.S. in 2001, 42 races were represented by a single isolate. Only six races were found occurring in more than a single region (Table 2). Race JBBL was found in both Texas and California; race LBBG was found in Alabama as well as in Texas and the Midwest. Four other races were found in both Texas and the Midwest. In 2000 the most common race groups in Texas were LB-- (14), LQ-- (12), and NQ-- (3), and the most common race groups in the Midwest were BQ-- (8), LB-- (4), LQ-- (30), and NQ-- (10 isolates). In 2001 the same race groups were prevalent, except that fewer isolates of the NQ-- race group were identified (Table 3). No doubt, this was due, at least in part, to contamination of the Cereal Disease Lab seed source for the Pc46 differential line during 2001. It is likely that 20-30% of the isolates identified as race group LQ-- in Texas and the Midwest in 2001 were really NQ--. LQ-- races are virulent on Pc40,. Pc38, and Pc39 but not Pc45, Pc46, Pc50, Pc48, or Pc68. NQ-- races are virulent on Pc40, Pc46, Pc38, and Pc39 but not Pc45, Pc50, Pc48, or Pc68.

Reference

Chong, J., Leonard, K.J., and Salmeron, J.J. 2000. A North American system of nomenclature for Puccinia coronata f.sp. avenae. Plant Disease 84:580 585.

Table 1. Estimated losses in oat due to rust in 2001

				Losses due to rust
		Yield in	Production	Stem rust		Crown rust
	1000 acres	bushels	in 1000		1000		1000
State	harvested	per acre	of bushels	Percent	bushels	Percent	bushels
AL	NA¹	NA	NA	T²	T	0.5	_
AR	NA	NA	NA	0.0	0.0	0.0	0.0
CA	15	60.0	900	T	T	T	T
CO	32	60.0	1,920	0.0	0.0	0.0	0.0
GA	35	65.0	2,275	0.0	0.0	T	T
ID	20	68.0	1,360	0.0	0.0	0.0	0.0
IL	40	80.0	3,200	0.0	0.0	T	T
IN	16	80.0	1,280	0.0	0.0	0.0	0.0
IA	130	70.0	9,100	0.0	0.0	T	T
KS	40	53.0	2,120	0.0	0.0	T	T
LA	NA	NA	NA	0.5	-	2.0	_
MI	55	64.0	3,520	0.0	0.0	0.0	0.0
MN	210	60.0	12,600	T	T	1.0	127.3
MO	20	50.0	1,000	0.0	0.0	0.0	0.0
MT	60	40.0	2,400	0.0	0.0	0.0	0.0
NE	60	61.0	3,660	0.0	0.0	T	T
NY	80	69.0	5,520	0.0	0.0	T	T
NC	30	56.0	1,680	0.0	0.0	T	T
ND	240	62.0	14,880	0.0	0.0	2.0	303.7
OH	85	73.0	6,205	0.0	0.0	0.0	0.0
OK	10	38.0	380	0.0	0.0	0.0	0.0
OR	25	77.0	1,925	0.0	0.0	0.2	3.9
PA	115	65.0	7,475	0.0	0.0	0.0	0.0
SC	25	57.0	1,425	0.0	0.0	T	T
SD	130	60.0	7,800	T	T	3.0	271.2
TX	160	45.0	7,200	0.5	36.4	0.5	36.4
UT	6	65.0	390	0.0	0.0	0.0	0.0
WA	12	55.0	660	0.0	0.0	0.0	0.0
WV	NA	NA	NA	0.0	0.0	0.0	0.0
WI	195	64.0	12,480	0.0	0.0	T	T
WY	28	42.0	1,176	0.0	0.0	0.0	0.0
Total of above	1,874	61.1	114,531		36.4		742.5
U.S. % Loss				0.03		0.64
U.S. Total	1,905	61.3	116,856
¹NA = Not Available, therefore, not included in loss totals.
²T = Trace

*Table 2. Races of Puccinia graminis* f. sp. avenae from oat in 2001**
		Number of			Percentage of isolates of NA race¹
State	Source	Collections	Isolates		NA-5	NA-10	NA-27	NA-29	NA-67
AL	Field	2	6					100
CA	Nursery	2	6	50		50
	A. fatua	3	8	88		13
KS	Field	1	3					100
LA	Nursery	2	6					100
MN	Field	1	2					100
	Nursery	8	23				13	65	22
ND	Nursery	1	3					67	33
SD	Field	1	3				100
	Nursery	1	3					100
	A. fatua	2	6				50	17	33
TX	Field	1	3					100
	Nursery	19	58				31	57	12
	A. fatua	1	3						100
U.S.	Field	6	17				18	82
	Nursery	33	99	3		3	21	60	13
	A. fatua	6	17	41		6	18	6	29
	Total	45	133	8		3	20	56	14
¹ SeeMartens et al., Phytopathology 69:293-294.

Table 3. Frequencies of virulence in oat crown rust isolates collected in 2001 compared to collections in 2000 and mean virulence frequencies for 1991-2000

	Frequency of virulence, %
	2001		2000		mean, 1991-2000
Diff.	Texas	Midwest	Texas	Midwest	Texas	Midwest
Pc14	95	82	98	88	88	87
Pc35	86	48	90	36	61	41
Pc36	75	88	67	55	48	48
Pc38	27	64	48	84	31	62
Pc39	27	67	50	81	29	53
Pc40	95	73	98	77	88	72
Pc45	14	3	5	3	12	4
Pc46	14	0	24	27	31	34
Pc48	7	12	0	1	7	6
Pc50	5	3	5	6	26	10
Pc51	73	48	79	49	73	51
Pc52	5	18	0	4	5	5
Pc53	9	3	24	0	5	1
Pc54	14	6	21	10	25	14
Pc56	66	82	60	47	49	48
Pc57	40	22	35	28	33	21
Pc58	27	6	17	3	21	7
Pc59	45	48	63	21	43	26
Pc60	86	27	86	45	78	55
Pc61	86	27	62	35	69	40
Pc62	5	9	0	8	2	5
Pc63	16	55	43	74	24	50
Pc64	5	9	26	10	8	8
Pc67	67	30	50	32	56	41
Pc68	2	0	2	3	2	1
Pc70	67	67	60	57	32	39
Pc71	24	76	48	76	28	52
No. isol.	44	33	42	77	663	751

Table 4. Oat crown rust races identified in two or more regions of the United States in 2001
	Number of isolates of each race
Race	Texas	Midwest	Alabama	California
JBBL	1	0		1
LBBG	1	4	2
LBLG	6	1
LLMG	1	1
LQLB	1	1
LQMQ	1	1
Total isol.	44	33	2	2

Table 5. Most frequent oat crown rust races and race groups identified in Texas and the Midwest in 2001

Texas		Midwest
Race/group	No. isol.	Race/group	No. isol.
LBBB	3	BQLQ	2
LBLB	4	LBBG	4
LBLG	6	LQGG	2
LBPG	5
B--- races	0	B--- races	8
L--- races	32	L--- races	23
N--- races	4	N--- races¹	0
Q--- races	4	Q--- races	1
BQ-- races	0	BQ-- races	5
LB-- races	25	LB-- races	5
LQ-- races	5	LQ-- races	10
¹In 2000, approximately 25% of isolates from Texas and 30% from the Midwest were N--- races. The lower proportion of N--- races in Texas and the absence of N--- races in the Midwest in 2001 is likely due to contamination of the Pc46 differential seed source sometime during 2001. Therefore, the L--- races listed in the table may contain a few isolates that should have been classified as N--- races. In 2000 there were 1/3 as many isolates of NQ-- races as LQ-- races in the Midwest.