Impact of nitrogen sources on spring dead spot

In a post earlier this spring, I briefly mentioned our recent findings on the impact of nitrogen sources on spring dead spot development in bermudagrass turf. I am preparing to present this research at the upcoming American Phytopathological Society Meetings in Portland, OR and thought I would provide everyone with a sneak peek of our findings.

Most turf managers in the southeast and midwest are all too familiar with spring dead spot. However, in light of the results below, it is important to realize that the disease is caused by three different species: Ophiosphaerella herpotricha, O. korrae, and O. narmari. The most common species in the southeastern US is O. korrae, whereas O. herpotricha is prevalent in the midwestern US. If you are unsure of which species you have, ask your friendly neighborhood turf pathologist to find out which species is most common in your area.

We initiated this research in 2004 to determine how fertilization programs influence these different spring dead spot species. Each plot was inoculated with O. korrae and O. herpotricha in Fall 2004, fertilization treatments were initiated in May 2006, and were continued through 2007 and 2008.

Some of the main findings of this research are as follows:

1. Spring dead spot caused by O. herpotricha was suppressed very effectively in all 3 years by fertilization with ammonium sulfate. Sulfur coated urea provided some control in 2008, and calcium nitrate provided moderate suppression in 2009.
2. Ammonium sulfate had no effect whatsoever on spring dead spot caused by O. korrae. Instead, calcium nitrate provided almost complete control of this species in all 3 years.
3. Fall applications of potassium, dolomitic lime, gypsum, or elemental sulfur had no effect on either spring dead spot pathogen.

What does this mean? We are unsure at this time if the observed spring dead spot suppression is due to changes in soil pH or other nutritional effects. For example, suppression of O. korrae by caclium nitrate may be due to higher soil pH or increased calcium availability. Regardless, if you have struggled with spring dead spot in the past, take a look at the nitrogen source you've been using and consider a change to either calcium nitrate or ammonium sulfate.

Many turf managers apply potassium in the fall, as this has been thought to help reduce spring dead spot development. During the three years of this study, we saw no benefit from fall applications of potassium or other nutrients. It is important to point out, however, that we applied 2.7 lbs K/1000 sq ft from potassium chloride (0-0-60) during the summer in conjunction with our nitrogen applications. Therefore, as long as adequate amounts of potassium are applied during the season, additional fall applications do not appear to influence spring dead spot development.