Friday, February 23, 2024

Leaf Spot Look-a-likes

Abiotic leaf conditions abound, and misidentification can be costly.

⋅ BY DANIEL J. LEONARD ⋅
Calhoun County Extension Director, Agriculture, Horticulture & Natural Resources Agent, UF/IFAS

With maturity still 30 to 40 days away in most area peanut fields this past season, detection of early leaf spot in multiple counties set Florida Panhandle growers on their toes, and with good reason. Finding early leaf spot before the more normal late leaf spot infections is yet another thing for them to worry about. Fueling these concerns were weather conditions favorable for disease development – extremely hot and humid with intermittent rainfall. The final puzzle piece that made growers’ lives difficult was the presence of abiotic or non-disease leaf spots that can be prevalent and muddy the waters when weighing an extra fungicide spray if not accurately identified.

A Field Visit

Chemical burn on peanut leaves. Note the light-colored centers and lack of sporulating structures.
Photos by Daniel Leonard, UF/IFAS

Because of the heightened awareness and favorable weather, I was called out to scout a Calhoun County field of TifNV-High O/L peanuts because of the grower’s concerns regarding leaf spot. The field in question was planted around 100 days prior and had plenty of time left for a pathogen to cause serious damage and yield loss.

Closer inspection revealed near-black spots in the lower canopy, but many were very irregular, unlike the typically circular early leaf spot, and didn’t possess sporulating structures you would associate with a fungal infection. Though an exact diagnosis is difficult with the many abiotic leaf conditions, these widespread, large, irregular spots appeared to be irregular leaf spot, also called funky leaf spot.

This picture shows non-sporulating leaf spots in the interior peanut canopy typical of an abiotic issue.

First found back in the late 1990s, irregular leaf spot looks similar to early leaf spot but generally appears earlier and is much more irregular in shape. Most importantly, it is not a pathogen, doesn’t cause the devastating lower-canopy defoliation of true leaf spot diseases and does not affect yield. There is no need for an additional spray for these abiotic leaf conditions if fungicides effective against leaf spot are already present in the spray program.

Significant Savings

In the same field, there were other spots that appeared more like late leaf spot at first glance – more regular in shape and dark brown. Again, after a closer look, they didn’t fit the profile of a fungal leaf spot. These spots, while more regular in shape, only appeared in the upper canopy, did not possess the under-leaf sporulating fungal structures and had a light brown to gray center. The presence of leaf spotting in the upper canopy with lighter, buckskin-colored leaf centers is a tell-tale sign of chemical burn. In this case, the spots were likely from a Provost Silver application a couple of weeks prior. Far from a bad thing, these abiotic lesions tell the grower that the fungicide is there and doing its job!

While it can be a race to protect peanuts from leaf spot pathogens, it’s important to remember that not every leaf blemish is cause for concern. Timely and accurate identification of field issues like leaf spot, which local UF/IFAS Extension agents are willing and able to help with, is critical and can either save your crop by heading off an outbreak or, in this grower’s case, prevent an unnecessary spray and expense, plus undue worry. PG

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