Rust Update 6/23/11

June 23, 2011

Xianming Chen
Wheat stripe rust

With the relatively warm weather in the last couple of weeks, stripe rust of wheat has developed fast in most part of the PNW, especially in the Palouse region (Whitman County, WA and Latah County of Idaho). Winter wheat ranges from late jointing stage to soft dough stage (Feeks 8 to 11) and spring wheat ranges from tillering to late boot stage (Feeks 3 to 10). Susceptible entries of winter wheat in our experimental plots (ranging from boot to flowering stages) around Pullman have 100% severity on all leaves and have produced a thick layer of rust spores on the ground. Susceptible entries of spring wheat also produced spores on lower leaves and the spore layer on the ground is noticeable. No artificial inoculation was used in our experiment fields. The rust severity is much higher than that of last year at this crop growth stage. Most of commercial winter wheat and many spring wheat fields have been sprayed once, twice, or even three times. Rust infection can be seen in distance in some fields or spray-missed stripes and corners.

Now is the critical time for the last chance of fungicide application for winter wheat fields with significant infection or fields sprayed over three weeks ago and new infections appearing. For spring wheat, it is the time to spray fungicide mixed with herbicide in many areas. Based on the current forecast, temperatures in the next 10 days will continue to be very favorable for stripe rust development in the Palouse region (night lows between 42 to 55oF and day-time highs from 64 to 79 oF) and many other areas in the Pacific Northwest. Even the temperatures in the Walla Walla area (night lows ranging from 51 to 61 oF and day time highs from 73 to 86 oF) in the next 10 days will not be able to stop stripe rust development. The infection during and after the rains last week in many areas will show up in a week or so. The good moistures in flat and low part of fields will continue producing dews on plants during night, which will allow new infection to occur every night for a week or so even without additional precipitations. Therefore, stripe rust is expected to continue increasing for two weeks.

High-temperature adult-plant (HTAP) resistance has been working, but not to its highest capacity under such low temperature and high-inoculum load. Based on the stripe rust data collected these days, the resistant to moderately susceptible reactions (infection types) of individual PNW winter wheat cultivars are basically the same as those of last year. However, some cultivars have higher severities even with a resistant reaction. Noticeable changes occurred on varieties ‘Simon’ and ‘George’, which were moderately resistant or resistant last year, but become susceptible this year. Because the much longer rust season and currently extremely high rust inoculum load, many resistant varieties may be worthy to be sprayed with fungicides. Based on our current disease data, resistant varieties like ‘Madsen’, which had 1% yield loss (not statistically significant ) last year, may have 5% yield loss this year. The susceptible check (‘PS 279’) will likely to have more than 70% yield loss. To fresh the memory of last year’s yield loss on major PNW wheat varieties, I attach the last year’s data again. For a rough estimation of potential yield loss on each individual variety, I suggest to add 5% yield loss for this year’s potential. To determine whether a field should be sprayed (or sprayed again) or not, please check your fields. If you see active rust pustules, consider to spray. If no rust or just short necrotic (dead) stripes, the field may not need to spray. Similar to last year, we have two major groups of races represented by PST-114 and PST-116 as one group and PST-127 and PST-139 as another group. The PST-114 group is virulent and the PST-127 group is not virulent on several hard red winter wheat varieties (such as Buchanan, Finley, and Farnum) because of the race-specific Yr10 and other resistance genes. These used-to-be susceptible (Buchanan and Finley) varieties are highly resistant in some locations this year, similar to last year. Therefore, a cultivar can be highly resistant in one location, but highly susceptible in another location; and it is important to check your field to make decision on fungicide application.

In some areas, heads of winter wheat have been infected, especially in irrigated fields. Head infection may reduce yield and quality, but generally the yield reduction is limited and also too late to use a fungicide. Reduce the amount of irrigation and increase interval between irrigation may reduce damage.

I have heard or been asked about the issue of fungicide tolerance. So far, there is no solid evidence of any fungicide significantly lost effectiveness. Some of failures would likely be due to use of a low rate or applied under a not right condition (e.g. too windy, too cold, wash off by immediate rain, not use a right surfactant, etc.). The efficacy and duration of effectiveness of currently labeled fungicides and their ranks are still the same as I have provided previously. I have noticed that some chemical is labeled at a half rate at the time of herbicide application and a full rate of flag leaf stage. We do not recommend using a half rate at any time of application especially under this year’s situation of an early-started long rust season. Therefore, use a fungicide more suitable for your crop may better protect your crop and get a better return.

Another question is whether rust-infected wheat straw is toxic to livestock or not. So far, there has been no study on such issue and also no any solid report or evidence for association of infected straw with livestock sickness.

Barley stripe rust

In recent days, we have found only very low stripe rust on barley in our nursery near Pullman. Barley stripe rust is unlikely to be a problem as the inoculum is very low in the Palouse region.

Stem rust

Barberry bushes in the Palouse region have been infected by stem rust and the infection has occurred later than last year. We expect to see spotted stem rust in wheat and barley fields in very late of the growing season in this region as the late crop is favorable to stem rust infection. However, the widely use of fungicides to control stripe rust will reduce stem rust infection for the period of time when crops are protected by the fungicides.