Bacterial Spot on Tomato: Are the Cures as Bad as the Disease?

From Clemson Plant Pathologist Tony Keinath and Extension Agent Zack Snipes.

Now that tomatoes are in the ground in coastal South Carolina, it’s time to think about a tomato spray program for a disease that shows up every year: bacterial spot. Current recommendations are based on a tank mix of copper + mancozeb. The comments below are not a hard-and-fast recommendation, but rather something to consider, as we look at “evidence-based” recommendations. In other words, what works?

Typical symptoms of bacterial spot on tomato leaves.

Actigard is registered on tomato at 0.33 – 0.75 (dry) ounce per acre to manage bacterial spot and bacterial speck, a closely related disease that occurs more often in cooler climates, like the Midwest, or once in a while in cooler-than-usual summers here. The label includes a range for gallons of water per acre based on the size of the plants.

In an on-farm trial done in Charleston County in the fall of 2017, Actigard was compared to another resistance-inducing product, Leap (Table 1). All three sprayed treatments reduced bacterial spot by about 50%. Tomato yields were 15% lower in the unsprayed treatment than in the Leap treatments (Fig. 1).

Yields also were 17% lower in the Actigard/ManKocide treatment than in the Leap treatments. Medium-sized tomatoes were the size most affected. Note that all plots received two applications of Actigard at the low rate, the first two applications. In this case, the cure (Actigard) caused as much yield loss as the disease (bacterial spot in the nonsprayed treatment). A similar yield drag with Actigard, a 27% loss, was observed on tomato BHN 602 in Florida in 2012 to 2014.

Figure 1. Yields of tomatoes treated with Actigard or Leap, Charleston County, SC, fall 2017.

Actigard is not the only product to potentially reduce tomato yields. In a trial in the mountains of North Carolina in the summer of 2019, two formulations of Nordox, Nordox 75 (copper oxide) and Nordox 30/30 (copper oxide plus zinc oxide), also reduced yields compared to the unsprayed treatment (Table 2). In this study, the unsprayed plants had the most bacterial spot but also the greatest yield. The North Carolina grower standard, Kocide + Manzate alternated with Actigard + Manzate, had the least bacterial spot and a 3% lower yield than the unsprayed treatment.

Treatments (amount per acre)% DiseaseTotal yield (lb/A)
Nordox 75 (Cu), 1 lb   63AB40,120
Nordox 30/30 (Cu/Zn), 1.5 lb53BC40,003
Kocide 3000 DF, 1.75 lb + Manzate Pro-Stick 75DG, 1.5 lb / Actigard 50WG, 0.5 oz + Manzate48C44,306
Table 2. Results of a bacterial spot trial, Mills River, NC, 2019

Tomato yields are difficult to estimate from small plots. Note that the yield reductions illustrated here are not verified statistically; they are trends. But it makes me wonder why pathologists, who tend to favor disease in their trials, cannot demonstrate a significant yield reduction by bacterial spot. And that leads me to the question, with a risk of yield loss from some spray products as great as the risk of yield loss from bacterial spot, why spray for bacterial spot?

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