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Disease pressure climbs with warming temperatures

From the June 20 Turf iNfo blog from University of Nebraska-Lincoln:

The recent wet weather and hot temperatures
in the forecast have us thinking about turf disease. The Smith-Kerns Dollar
Spot model is approaching 50%, a very high risk threshold, and we’ve received
reports of leaf spot on ryegrass and Kentucky bluegrass lawns, fields, and
fairways. The conditions are primed to see these diseases continue and for
mid-summer diseases to appear.

Brown Patch

Brown patch in tall fescue and creeping bentgrass is a perennial problem during summer in the central Great Plains. High heat and humidity favor this disease. While brown patch rarely causes long-term issues, it can be unsightly to some. Try to encourage leaf drying and consider a preventative application of a fungicide if an outbreak is completely unacceptable. Here is a great resource for selecting the best disease control product from Drs. Vincelli and Clarke: Chemical Control of Turfgrass Diseases.

While nitrogen fertilization is often
linked to brown patch severity, the latest research suggests this connection
isn’t as strong as once thought. Under-fertilized tall fescue lawns can even
have worse disease symptoms because the plant cannot “grow-out” of the damage.
When it comes to fertilization in summer, don’t fertilizer if the grass is
already green and rapidly growing. This can make the disease worse. However, a
fertilizer application in mid-summer can often help lawns with minimal growth
in the summer. Consider a slow-release source, like an organic fertilizer, to
minimize the risk of fertilizer burn and be sure to water-in the fertilizer
after.

Anthracnose

As the stress on the turf increases, the
risk of anthracnose is also enhanced. While anthracnose can occur under diverse
environmental conditions, high humidity and temperature following late spring
flowering can lead to rapid disease development. Stress exacerbates disease
progression. Practices that promote plant health can help with anthracnose. On
putting greens, those practices include raising the mowing height, withholding
aggressive verticutting, light and frequent sand topdressing, and application
of sufficient nitrogen to sustain adequate growth. In annual bluegrass, light
spoon-feeding of potassium fertilizer can help reduce anthracnose (tissue K
>2%). Also, use soil moisture meters to precisely apply water to the turf.
Still preventative applications of fungicides are typically required to prevent
this damaging disease from causing problems.

Pythium

Pythium blight is
feared by many turfgrass managers. We all have heard stories of greens killed
overnight by this disease. While it can cause catastrophic damage, it is more
common on high mowed creeping bentgrass and ryegrass (fairways) that have
received too much water when the air temperature and humidity are very high.
And still, damage is usually isolated to low-lying areas unless it is moved by
mowers or by water flowing over the surface. Curative applications of
fungicides can help managers defend against this damaging pest.

More common Pythium diseases are the Pythium root diseases. These pests are much less dramatic than Pythium blight, but can still have a severe impact on turfgrass quality. These root diseases reduce water and nutrient uptake and leads to slow turf decline. Wet soils favor Pythium root diseases. It is tempting to add more water to these areas, but that can actually make the disease worse. In fact, Pythium root rot researchers typically run light irrigation during the afternoon to promote the disease. Practices to maximize turf health like venting, adequate nitrogen fertilizer, and use of wetting agent can help, but fungicide applications are typically needed when the soils are wet and have significant organic matter. One of the most commonly used fungicides for Pythium root diseases is cyazofamid (Segway); a Qil active ingredient (FRAC 21). As with other fungicides, it’s important to rotate chemical families to avoid disease resistance. The QoI (strobi) fungicides, FRAC 11, are frequently used in rotation with Segway. They include active ingredients like azoxystrobin (Heritage), fluoxastrobin (Fame), and pyraclostrobin (Insignia). Products like ethazole (Terrazole), fosetyl Al (Signature), and propamocarb (Banol) are also effective on Pythium rot root. These products should be watered in to achieve best results. North Carolina State University has great information and specific control recommendations for both Pythium root rot and Pythium root dysfunction.

Iron deficiency

While not a biotic disease, iron chlorosis is common in Kentucky bluegrass and creeping bentgrass, when soils are warm, wet, and have a high pH. Nitrogen fertilization actually makes this yellowing worse as the leaves grow faster but the plant can’t make enough chlorophyll to provide a dark green appearance. It’s best to preventatively treat these areas with foliar iron fertilizer. It can be from dissolved iron sulfate or liquid chelated iron sources. Our research suggests they all work about the same, and more iron (Fe) led to more greening regardless of the iron source. Granular iron fertilizer does not help, because struggling turfgrass roots cannot take it up. Be careful with very high rates of iron on hot days to minimize the risk of burn. Do not water-in the iron fertilizer. Here’s a past TI on the subject: Observations and treatment of iron deficiency and chlorosis.

Bill Kreuser, Assistant Professor and
Turfgrass Extension Specialist, wkreuser2@unl.edu