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Unusual September Weeds and Diseases in Iowa

September 24, 2013

With last week’s abnormally warm September weather, several diseases and weeds were discovered around the state of Iowa. Brown patch thrived with night temperatures above 70F⁰ coupled with high humidity. Brown patch is caused by the fungal organism Rhizoctonia solani. Damage affects the leaf blade from the tip down and is usually noticed in grasses which receive high amounts of nitrogen fertilization. Symptoms are straw colored irregularly shaped foliar lesions with a brown boarder. R. Solani can attack most cool-season grasses, but is most commonly noticed on creeping bentgrass greens, tall fescue lawns and Kentucky bluegrass.

Symptoms on bentgrass putting greens appear as a copper/gray-colored “smoke rings” ranging from a few inches to several feet where mycelium can be seen. Figure 1 below was taken last week by Dan Strey at the ISU research station. Figure 2 is from University of Missouri Extension IPM: Identification and management of turfgrass disease - looking at leaf and sheath lesions of brown patch.  


Figure 1: Brown Patch at ISU research station
Figure 2: University of Missouri publication looking at tall fescue foliar syptoms of brown patch


There are many fungicides that provide brown patch control such as Daconil, Banner Maxx, Heritage, and several others. Cultural practices such as reducing nitrogen levels and preventing long periods of wet conditions can reduce disease pressure. With temperatures tapering off over the weekend, hopefully it will be the end to the high temperature summer diseases in Iowa.

Oddly enough, in the middle of September we have also seen crabgrass and goosegrass seedlings germinating at the ISU research farm. Normal crabgrass germination occurs in mid-April to mid-May depending on your location in the state. Crabgrass is easily identified with fine hairs on the leaves and sheaths as well as its distinctive “protruding fingers” seedheads. Crabgrass also has a rolled vernation, while goosegrass has a folder vernation. 

Goosegrass is often mistaken for crabgrass and some people incorrectly refer to it as “silver crabgrass” because if it’s silvery appearance of the lower sheaths.  Goosegrass generally germinates 2-3 weeks later than crabgrass in the spring. The seed stalks of goosegrass also appear somewhat like a zipper with two individual seeds protruding in two directions. In figure 3 and 4 below you will see the side by side comparison from the Scotts grass manual.

Figure 3 and 4: Goosegrass and Crabgrass comparison from Scotts grass manual

Goosegrass is very difficult to control, even with the use of preemergence herbicides. The best postemergence option is a nonselective herbicide such as glyphosate. Optimal crabgrass postemergence control is obtained when applied while the crabgrass is small and actively growing. The use of fenoxyprop, quinclorac, and dithiopyr are the best options.




December 20, 2010

Here is the 3rd post by undergraduate student Steve Johnson on his pathology project at ISU during the summer of 2010.

Steve Johnson, Soph. Summer Intern Blog #3

Following my last blog I will discuss the results of the experiment.

Assessments of the turf were made by my instructor Mark Gleason, Professor of Plant Pathology and Horticulture at Iowa State University. Data were recorded on 2 July, 15 July, 26 July, and 18 August at the Turfgrass Research Area of the ISU Horticulture Research Farm near Gilbert, IA, and at a green near the WOI Building on the ISU campus. Dollar spot was recorded as a percentage of the area in a 5-ft x 4-ft plot that was covered with the disease; all treatments had 4 replicate plots, arranged in a randomized complete block design. Turf quality ratings were set on a qualitative scale of 1 to 10 where 10 indicated no disease, excellent quality, and a 1 indicated very poor turf quality. Data were analyzed using the GLM procedure and SAS (statistical analysis software) with mean separations determined by Fisher’s protected LSD at P<0.05.

Weather conditions for the 2009 summer hit record highs in heat, rainfall, and humidity. Heavy rainfall caused extensive flooding in the Ames area which persisted from 11-13 August. There were no signs of phytotoxicity seen on the turf for the duration of the trial for either location. However, worth noting is that all four sub-plots for treatment 11 (a pre-mix of chlorothalonil and propiconazole) at the Hort Farm displayed a darker green coloration and sometimes slight browning on 15 and 26 July.

At the Hort Farm, dollar spot was light to moderate in disease intensity over the course of the summer. Intensity peaked in late July with a decline occurring by August. However, due to variation among subplots, most treatments did not vary significantly from the untreated control. Turf quality had similar results, with most of the treatments showing a consistent decline in the quality as the summer progressed.

However, many fungicide treatments exhibited significant difference in dollar spot severity on 2 July and 26 July. In addition, a few of the treatments maintained good quality the entire summer, indicating that that these treatments proved effective against dollar spot and preserved adequate turf visual quality despite the stressful growing conditions.

For WOI, the data were in question due to a severe outbreak of crabgrass. Creeping bentgrass at the location was overwhelmed to such a degree that WOI will not be used again for future experiments. There were two reasons for this. Golf course maintenance was inhibited due to tree damage from a storm in mid July that produced 70-mph winds, as well as severe flooding from 11-13 August. While I have made available the data in Tables, the results are questionable for WOI.

I have attached 4 Tables showing the data that were collected on check dates over the summer. Data Tables include: dollar spot % severity at the Hort Farm and WOI, as well as turf quality for the Hort Farm and WOI. On 26 July, Mark and I independently assessed % dollar spot severity at the Hort Farm. As was explained in the earlier blogs, this was to improve the reliability of the disease % ratings of dollar spot on turf by combining the impressions of two raters. The data was averaged between Mark and myself and was recorded under 26 July column for the Hort arm % dollar spot Table.

Despite the numerous and overwhelming weather issues and outburst of crabgrass at WOI, this experiment still yielded some good data concerning the effectiveness of fungicide treatments. Also valuable is the method of averaging disease ratings from multiple raters to reduce individual biases. The amount and reoccurrence of fungicide sprays are factors determined by accurate readings which can save money and resources as well as prevent over-applications of fungicides which can lead to phytotoxicity of grass blades. It is methods and good data learned from studies such as this one that can prove quite useful in telling how well established a disease is and aid owners in deciding upon a proper integrated disease management program for optimal disease prevention.