ISSUE 13 July 29, 2004
FUNGICIDES FOR SOYBEAN
The main foliar and stem disease of soybean present in North Dakota is white mold (Sclerotinia stem rot). Although, in some cases, white mold can reduce soybean yield, rarely does it justify spraying a fungicide for control. Soybean plants can compensate for yield quite well; therefore it usually takes a minimum white mold incidence of 30% before an economic yield loss in a soybean field is observed. Due to the nature and biology of the Sclerotinia fungus, the main site of infection on soybean is the flowers. In a solid-seeded soybean field, penetrating the canopy with a fungicide and getting adequate coverage on the flowers can be very difficult. The indeterminate trait of the soybean varieties grown in North Dakota makes fungicide timing difficult, because there are always new unprotected flowers appearing that provide a site of infection for the Sclerotinia spores. If the decision to spray for white mold on soybean is made, the only fungicide registered for control in soybean is Topsin M (and generic formulations of thiophanate-methyl).
Other fungicides registered for control of other soybean diseases, such as Septoria brown spot and pod and stem blight, include Quadris and chlorothalonil products (Bravo and others). These other diseases generally do not cause any economic losses to North Dakota soybean fields.
Iíve received a few questions about making a Quadris + Warrior (insecticide) application late in the season. Apparently, some data has shown that there may be a yield enhancement with this application in some states. Because of the potential of developing fungicide-resistant pathogen populations with the class of chemistry (strobilurins) in which Quadris belongs, making an unwarranted decision to blanket-spray soybean fields is risky. In general, foliar fungicides for soybean grown in North Dakota are not currently needed. If the soybean rust disease ever enters the United States, then fungicides may be needed (see below).
Fungicides for soybean rust. Although soybean rust is not present in the continental United States, a quarantine section 18 emergency exemption has been requested for several additional fungicides for soybean rust control. If the disease is ever found in the U.S., the fungicides approved by EPA in the section 18 request would be available for use.
Extension Plant Pathologist
NDSU IPM SMALL GRAINS SURVEY UPDATE, JULY 19-23
The NDSU IPM field scouts surveyed 121 wheat and 46 barley fields in North Dakota the week of July 19 to July 23. With wheat, 76% of the crop surveyed was in the milk to dough stages of development, while 89% of the barley fields surveyed were in the milk to dough stage of development.
In wheat, fungal leaf spots are increasing in severity, with some flag leaf severities as high as 30% observed in the more mature fields in the southeast. Wheat leaf rust was commonly observed in fields in the southeast, central and north central and northeast counties. Severities of wheat leaf rust are generally in the 1-15% range on the flag leaf. Fusarium head blight (scab) was observed in 20% of the post-flowering wheat fields surveyed, but field severities observed were generally less than 1%.
In barley, fungal leaf spots of Septoria, net blotch and spot blotch were observed, but at low levels in most fields. Only 13% of the barley fields surveyed had some level of Fusarium head blight symptoms, with average field severities of less than 1%.
Loose smut was observed in wheat and barley. Most fields with symptoms had incidence of less than 1%, but 4 wheat fields had loose smut incidences between 16 and 30%. This is a very high percentage of the crop to lose to a disease that can easily be controlled with proper seed treatment.
Bacterial leaf streak has been observed in scattered wheat fields in the east. The bacterial infection causes a long, brown necrotic area on the leaf that often is still shiny with dried bacteria. In some cases, the bacterial streak occurs in pockets or patches in the field. Rain storms with high winds allow wounding of the leaves and the wounds allow bacterial entry. No fungicide treatment will control bacterial leaf streak.
CEREAL RUST NEWS
The latest edition of the Cereal Rust Bulletin (July 23, 2004) from the USDA Cereal Disease Lab in St. Paul, MN, indicates that wheat leaf rust is severe this year on susceptible varieties and concentrated in the upper Midwest.
The hot temperatures in mid-July stopped the development of stripe rust on spring wheat in the Northern Great Plains. Our observations in ND also indicate that the low level of stripe rust infections apparent in some fields in early July have not developed further.
A few reports of wheat stem rust occurred in the southern and central plains states, but all the current spring wheat cultivars are resistant to the current race populations. I found a trace of stem rust on Norstar winter wheat in plots near Lisbon on July 19th. Norstar is a winter wheat with susceptibility to stem rust.
Reports of oat stem and crown rust are primarily from SD, and MN and WI. Trace severities of barley stem rust and leaf rust also were reported from MN. A few barley fields with trace levels of leaf rust also have been observed in our ND survey.
Extension Plant Pathologist