Crabgrass is out in full force in 2016!

Why is crabgrass so bad this year?
Several factors contribute to a bad year for crabgrass.
  • Crabgrass is a warm-season grass meaning that it thrives in hot conditions with optimum growth in temperatures ranging from 88 to 95 °F. However, our primary lawn species in Indiana are cool-season grasses and they prefer cooler conditions and have optimum growth in temperatures ranging from 68 to 77 °F. Therefore, our warmer than average summer resulted in an environment that encouraged crabgrass growth and decreased the plant competition from our existing cool-season grass lawns. NOTE: One minor exception to this is that tall fescue lawns - the cool-season turfgrass species with the best heat tolerance – had less crabgrass than Kentucky bluegrass and perennial ryegrass lawns.
  • The preemergence herbicides (crabgrass preventers) that are applied in spring to prevent the emergence of crabgrass seedlings work only as long as they remain in the soil. Generally, these products last in the soil about 60-120 days depending on a number of factors (rate applied, ingredient, turf cover, temperature, moisture, etc.). They breakdown in the soil overtime due to microbial activity.  When soil temperatures are higher, as they were this year, the preemergence herbicides breakdown more quickly. Additionally, the adequate rainfall that we had this year kept the soil moist and the microbial populations high, which also assisted in the herbicide’s breakdown. As such, a hot, humid summer results in the herbicide lasting a shorter length of time and the crabgrass “breaking through”. NOTE: When we have wet springs, some preemergence herbicides breakdown more quickly in the saturated soils due to anaerobic breakdown in saturated soils. It is a myth that preemergence herbicides leach through soils. They do not. Instead they are strongly bound (adsorbed) to the soil (at the surface where the crabgrass seed lays).
  • This was a good summer for turf diseases. Read the recent update from Dr. Rick Latin on disease in 2016. Crabgrass is susceptible to some diseases including leaf spot, but is generally not susceptible to the lawn diseases that cause our turf to decline in hot, humid summers. As such, lawns suffered and crabgrass did not.
  • Many homeowners mow too low and as a result, crabgrass increases. Lawns mown at 3.0 inches or higher have far fewer crabgrass problems than lawns mown short. Therefore, raise that mower so that you’ll have less crabgrass in the future. BONUS: Lawns mown at a higher height need mown less frequently, have deeper roots, are healthier, and need less watering.
  • Lastly, crabgrass plants get pretty big by the end of the summer. One plant can cover an area as big as a pie plate. This makes the lawn look poor even when a preemergence herbicide was applied. Keep in mind that there are thousands of crabgrass seeds in the soil (millions in some lawns). Even when a preemergence herbicide controls 95% of the crabgrass plants in your lawn, it may still not appear to have worked that well in a year like this.


What to do next?
The good news is that the end is near. Crabgrass will get zapped with the first hard frost this fall and you can begin now to battle against it and keep it from being problematic next year. This fall you’ll want to
  • Fertilize your lawn (at least once, twice is better). Read more here. Fertilizing the lawn in the fall months will help increase turf density and allow the turf to be much more competitive with crabgrass the following year.
  • Seed thin areas of your lawn as soon as possible. Read more here.
  • Adjust your mowing height so that your lawn is cut at 3.0 to 4.0 inches tall to help prevent future crabgrass problems.
Should I attempt to treat the crabgrass now in September?
NO! This is a summer annual plant, meaning that it will die naturally soon with the first frost. Additionally, large crabgrass plants are very difficult to control with herbicides. Two herbicide applications are needed to kill large, tillered-plants. As such, it makes no sense to spray expensive, marginally effective applications on a plant that is about to die naturally.

More information about crabgrass:

Aaron Patton
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Lawn Diseases 2016

The summer of 2016 has been quite favorable for the development of an assortment of infectious diseases on residential turf.  The primary driver of all of this disease activity is the weather.  In the lower Midwest, the combination of elevated temperature and ample precipitation since the beginning August of has spawned outbreaks of brown patch, dollar spot, Pythium blight, leaf spot diseases, and the appearance of symptoms of summer patch and necrotic ring spot.  Although infection is almost entirely a function of temperature and moisture, there are other factors (addressed below) that may hasten the decline of turf once disease becomes established.

Turf species:  Not all turf species are susceptible to all diseases. 
In general, Kentucky bluegrass (KBG) is less prone to brown patch , dollar spot, gray leaf spot, and Pythium blight—but very susceptible to root diseases such as necrotic ring spot and summer patch.  Root pathogens are present in all soils, and infect turf as soil temperatures increase beyond 60F.  With ample moisture, infection will continue through summer.  Plants are killed outright in some instances, but in most cases, KBG can tolerate some infection.  However, during periods of environmental stress, infection-impaired plants cannot survive, and characteristic dead patches or rings or frog-eyes appear. 

After the droughty years of 2009-2012, many of us turned to turf type tall (TTF) fescue to better tolerate periods of dry weather.   The summers of 2015 and 2016 have revealed the weakness of these grasses in terms of susceptibility to gray leaf spot and brown patch.  They are not susceptible to root diseases-and that is an important reason for favoring this type of grass. 

This was not a good year for perennial ryegrass (PRG)!  Its only redeeming feature is that PRG is not susceptible to the root diseases.  We are learning that even the so-called “gray leaf spot resistant” cultivars will suffer during prolonged periods of favorable weather.

Fertility:  Nitrogen status will influence disease severity.
Some pathogens are favored by nitrogen levels that approach excess, others thrive when nitrogen levels are low.  I usually recommend a little (0.2 lb/M) supplemental N in early summer, when the fall-applied reserves have been used.  This helps keep plants growing through summer and helps recovery from red thread, dollar spot and actually can prevent serious rust outbreaks.  However, this practice can be risky on PRG and TTF because brown patch is favored by lush growth promoted by nitrogen.  Also, from my experience, supplemental N (0.25 – 0.50 lb/N) during summer can help mask the effects of summer patch and necrotic ring spot (on KBG only).  Again this can be risky because some leaf spot diseases (NOT gray leaf spot) flourish when leaf and shoot growth are stimulated by N.   Pythium blight, a disease that strikes when temperatures exceed 90F during the day and 70F during the night is also favored by high nitrogen nutrition.  This year seems to have been one where we are  “damned if we do and damned if we don’t” with regard to nitrogen fertility.

Irrigation:  Timing and amount.
We cannot define the best time to irrigate in all different scenarios, but we do know the worst time—early evening hours (6:00 pm - 9:00 pm).  Irrigation at this time adds to the period during which leaves are wet.  An increase in leaf wetness by a few hours can mean the difference between a severe outbreak and the appearance of some mild symptoms. It is difficult to generalize about the amount of water to apply.  However, from a disease prevention perspective, deep infrequent watering is better than light frequent irrigation.

Growth stage:  Pitfalls of over-seeding during disease-favorable weather
We tend to think about repairing damaged stands of turf in late August through September. Seedlings and juvenile plants are most susceptible to infection.  Normally, the risk of infection is acceptable because weather changes to a pattern that favors turf growth more than fungal growth.  Not this year. 

Fungicides:  Timing is everything (almost)
Fungicides are valuable chemical tools to help avoid or mitigate turf damage.  Granular products are available with multiple active ingredients to protect against a variety of diseases.  However, they work best when applied preventatively, or before symptoms appear.  Having said that, I am convinced that modern fungicides can be used to control dollar spot, brown patch, red thread, and rust--even after symptoms appear (but certainly long before the lawn is wiped out).   Leaf spot diseases (including gray leaf spot) are best controlled before symptoms show. When symptoms become apparent to most observers, infection is way beyond the stage of a “mild” infection.  Most of these fungi reside in the turf or thatch or soil, so the same areas of turf will be prone to outbreaks each year.  The weather will determine if a severe outbreak materializes. This year the weather favored disease.
 
The most difficult “fungicide” decisions involve the root diseases on KBG.  Fungicides can be effective, but they must be applied in late spring, often before there are any symptoms at all.  (We know that fungicides are NOT effective against root diseases once the patches and/or rings are in full form).  So, it is important to understand the disease history of the site before considering a fungicide option.  Caution--because fungicides do not move down to the roots through the leaves inside the plant, fungicides must be washed off leaves (into thatch/soil) for any chance of success.  Getting the right amount of chemical to the right place at the right time is not easy , and this difficulty further complicates decisions to use fungicides for controlling root diseases on residential turf.

Summary:
More than any time in the past 10 years, weather during the summer of 2016 has favored outbreaks of so many turf diseases.  Mercifully, fall is approaching and we can begin the process of repairing damaged lawns.  We should also take note of diseases that caused problems this year and the factors that influence their outbreaks. To be sure, when such weather patterns return in the future, the same disease threats will revisit our lawns!

For further information on turf diseases, visit our website at
https://ag.purdue.edu/btny/Extension/Pages/ExtPubs.aspx
or, use your favorite browser to search “Purdue Turfgrass Disease Profiles”.

Rick Latin, Ph.D.
Professor of Plant Pathology
Purdue University

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