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Know Your Fungicide Options And When To Use Them

• By Heather Marie Kelly •

Cotton is blooming in Tennessee, and I’ve gotten some calls asking about fungicides – there are options but none is warranted at this time. As the rain, heat and humidity continue, we may start to see target spot develop, which could justify a fungicide application.

Only since 2013, when target spot in cotton was first reported in Tennessee, has there been a benefit to applying foliar fungicide to cotton. Hence, it is very important to be able to identify target spot from other foliar diseases.

Resources at UTcrops.com and mobile friendly field guides at https://guide.utcrops.com contain diagnostic key for identifying different foliar diseases as well as specific information on each disease (symptoms, potential impact on yield, management options, pictures, etc.).

target spot in cotton

Target spot on leaf and bracts

Fungicides labeled in cotton (product rate fl oz/a): Amistar Top 8 – 11.6, Aproach 6 – 12, Delaro 8 – 12, Headline 6 – 12, Priaxor 4 – 8, Proline 5 – 5.7, Quadris (and other Azoxystrobin products) 6 – 9, Topguard 7 – 14, Topguard EQ 5 – 7, Tebuconazole products 6 – 8

Target spot develops in the lower canopy, usually only after canopy closure. It is more problematic in high yield situations and rank growth. As the name suggestions, the lesion on leaves look like targets, with concentric rings. Main factor of the disease that has correlated to yield loss is the defoliation it can cause from the lower canopy and upwards.

Based on Tennessee trial data from 2014 – 2017, only 15% of the time did fungicides significantly protected yield, and those times that fungicide are beneficial depend on variety susceptibility, disease pressure and environmental conditions (back to the factors that influence when a disease becomes a PEST – Pathogen presence, Environment, Susceptible host, and Timing – growth stage and calendar date).

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When factors warrant a fungicide application, later fungicide application timings had the best probability of decreasing defoliation and protecting yield (third week of bloom and/or fifth week of bloom, which have correlated to when lesions are found in the lower canopy) in Tennessee.

Of the products tested, Headline and Priaxor have had the most consistent results, but again only when disease is present, weather conditions are conducive for disease development and only when this aligns during certain time of year will fungicide be warranted in cotton.

Dr. Heather Marie Kelly is a University of Tennessee Extension plant pathologist. She may be reached at youngkelly@utk.edu.