Joel Ransom

NDSU Extension Agronomist - Cereal Crops

joel.ransom@ndsu.edu

With winter wheat in the middle of grain-filling and other small grains not that far behind, it is now time to think about harvesting. If green weeds in the crop will hinder the harvesting process there are several herbicides labeled for pre-harvest weed control (refer to the 2008Weed Control Guide and the labels of approved products for additional details). Weed control with pre-harvest herbicides, however, is generally disappointing as weeds at this time are tall, nearing maturity and slow growing. Furthermore, green weeds can take a week or more to dry down even with an effective treatment.

Glyphosate, in addition to controlling weeds, is labeled for use in aiding the dry down of the crop itself (as opposed to controlling and drying down weeds in the crop). Glyphosate is a systemic herbicide and takes from 7 to 10 days to effectively kill the growing parts of the crop, consequently an increased rate in dry down is not immediately visible. Traditionally, fields that had excessive green material were swathed. Swathing enables faster dry down than pre-harvest glyphosate if significant levels of green material are present in the crop. A standing crop that has been treated with glyphosate, however, will dry faster than a swathed field after a rain. When using glyphosate pre-harvest observe the following guidelines:

1. Apply glyphosate only after the crop has reached physiological maturity. For most varieties this occurs at a grain moisture content of about 30%. At this moisture content the grain is in the hard dough stage; if you run your thumb nail across the kernel, the indentation will remain. Applying glyphosate before physiological maturity can reduce yield, test weight and seed germination.

2. Because germination can be affected when applied too early, glyphosate should not be used in fields that will be used for seed or on barley intended for malt.

3. Pre-harvest applications of glyphosate must be made at least 7 days before harvest.

As with all agricultural chemicals, read and follow the label when using herbicides pre-harvest in small grains.

Marcia McMullen

NDSU Extension Plant Pathologist

marcia.mcmullen@ndsu.edu

Sam Markell

Extension Plant Pathologist

Downy mildew of sunflower has recently been observed in North Dakota. Although control of the disease at this point in the season is not possible, the observation of downy mildew is important for future years.

Downy mildew generally causes a systemic infection, which results in a severely stunted plant and death in the seedling stage. Although it can also cause a secondary infection, which occurs later and causes limited damage to plants. When plants are systemically-infected with downy mildew they will often be stunted and their growth will be distorted.

The leaves may appear ‘crinkly’ with yellowing on the top sides of the leaves (particularly along the leaf veins) with a white cottony growth on the underside. Secondary infections appear as small chlorotic (yellowed) spots on the leaves and do not result in a systemic infection or stunting.

Sunflower with a systemic downy mildew infection next to a healthy sunflower. Note: plant stunting with deformed and yellowing leaves.

Yield losses to downy mildew are generally low because systemically infected plants often die, and adjacent sunflowers are able to compensate by producing larger heads. In general, yield loss is not usually observed

if less than 5% of the plants are infected. However, when infected plants are in clusters or in rows the compensatory ability of sunflowers is reduced, and yield loss in that part of the field may be significant.

Secondary infection usually causes no yield loss. Once downy mildew occurs there are no tools to control it, thus, prevention of downy mildew is the best management strategy. The pathogen produces spores that may remain in the soil for many years, and when the environment is favorable, may germinate and cause infection for up to five years after downy mildew was first observed in the field. For this reason, it is important to take note of any downy mildew occurring in your fields. Several years of rotation, downy mildew resistant varieties, and seed treatments are recommended when returning to a field with a history of downy mildew.

Rich Zollinger

Extension Weed Specialist

r.zollinger@ndsu.edu

While herbicide advertising often talks about full-season weed control, for most weeds we only need herbicides to control weeds until the canopy has developed sufficiently to suppress any late-emerging flushes. Weed survival rates can be greater at earlier emergence dates but biomass and seed production of weeds emerging at mid-season can be suppressed more than 90% compared to weeds emerging with the crop.

Many fields are likely to have greater problems with late-emerging weeds than normal. There are two primary causes for these infestations: 1) dry and cold conditions in May will suppress activation of soil-applied and excess rain in June will reduce the length of control provided by residual herbicides, and 2) poor canopy development due to cool temperatures, saturated soils and reduced stands will provide a favorable environment for weed growth.

Fields should be carefully evaluated prior to spraying weeds in large crops. Weeds that emerge significantly later than the crop are at a competitive disadvantage with the crop due to the crop’s head start. While these weeds may survive and produce seed, their impact on the yield should be minimal unless they are thick enough to create a sod. For example, if you have 3 to 4 inch weeds in 30 inch corn it probably doesn’t warrant an additional weed control trip. Spraying late season crop may cause more damage to the crop than would be gained by eliminating late-emerging weeds. Most importantly, the majority of herbicide labels prohibit application to larger crop due to problems with crop tolerance or efficacy. Glyphosate has restrictions in RR crops also.

Everyone expects weed-free field at harvest, sometimes it is best simply to live with the hand that’s been dealt. While late-emerging weeds in most fields are unlikely to impact yield, the seed produced by these plants will increase weed densities the following year. This increase in weed populations should be taken into account

when developing weed management plans for 2009.

Joe Zeleznik, Extension Forester, joseph.zeleznik@ndsu.edu

I’ve begun to receive calls on ash rust, a common disease of ash trees. This fungus is often noticed as small patches of orange lesions on the leaves. Sometimes the disease attacks the petiole or twig, occurring as a large misshapen mass. The lesions contain two types of disease spores. If the infection is severe enough, leaves will fall off the tree prematurely.

 Photo by Joe Zeleznik.                                                                             Photo by Shelley Feist, Logan County Soil Conservation District

Most infection occurs in the spring, but new infections can still occur in early summer if conditions are right – wet weather with warm temperatures, about 55 to 75 F. The best way to control ash rust is to eliminate

nearby cordgrass. If this is not possible, fungicides that contain the active ingredient myclobutanil may be used as a protectant fungicide on the trees whenever conditions for severe infection are high.