Making Lemonade Out of Lemons – A Tomato Fungicide Stewardship Tip Amidst Changing Regulations

Cheryl Trueman (Ridgetown Campus, University of Guelph) & Janice LeBoeuf (OMAFRA)

It seems like recently there have been a rash of proposed or pending pesticide regulation changes that affect field growers, and tomato growers are no exception. There are re-evaluations ongoing for a number of products used in tomatoes, including mancozeb, neonicotinoids, and Lannate, as well as Ethrel, but the big one that comes to mind for field tomato growers is the proposed changes to the use of chlorothalonil (Bravo, Echo). The final outcome of this review is not yet known, but it’s likely that significant changes to the chlorothalonil labels are coming.

Chlorothalonil is a go-to fungicide for tomato growers. Data from trials at Ridgetown Campus demonstrate its value. Chlorothalonil is often just as good at controlling early blight, Septoria leaf spot, and anthracnose fruit rot as alternative fungicides, and it also provides protection from late blight, which many targeted fungicides do not. It’s a good value active ingredient for tomato disease management and has a low risk of resistance development. But, if proposed changes go through, the number of chlorothalonil applications you can use will be drastically cut.

So, have you thought about how you are going to adapt? Being forced to change our practices through regulation and enforcement can cause a range of reactions from annoyance and frustration to rage. But after the initial shock, maybe you can make lemonade from lemons and use your energy to re-evaluate your fungicide program.

One simple change to consider for this growing season is use of early season fungicide applications. With the high value nature of field tomatoes, it’s tempting to protect your investment at any cost and apply protectant fungicides well before canopy closure and fruit set. But, if we go back to basics and consider the disease triangle (Figure 1) and the environmental conditions required for successful infection by fungal disease pathogens (Table 1), it’s apparent that the risk from fungal diseases and late blight in the first weeks of the growing season is quite low. In June, our dew periods are relatively short and it’s not that wet or humid in the developing tomato canopy.

Processing tomato field in spring

If you’ve been applying chlorothalonil early season (i.e. before early fruit set) it might be time to reconsider this practice. At Ridgetown Campus, we’ve achieved very good anthracnose control when fungicide applications begin at early fruit set, and do not see early blight, Septoria leaf spot, or late blight develop until there is a closed canopy. Our observations are consistent with the science on the environmental conditions required for pathogen development.

Ending the practice of very early protectant fungicide applications could save you time and money, and you’ll be one step closer to adapting to the anticipated new regulations that limit the number of chlorothalonil applications to field tomatoes.

Figure 1. The disease triangle – plant disease results from the interactions among a susceptible host, a virulent pathogen, and a favourable environment.

Disease triangle

Table 1. Common foliar and fruit diseases of field tomatoes in Ontario and the environmental conditions that favour their development (adapted from the Compendium of Tomato Diseases & Pests, 2nd Edition, APS Press).

Disease Causal Agent Environmental conditions that favour development
Early blight Alternaria solani

Alternaria tomatophila

Optimum temperature 24-29°C + rainy weather or heavy, long dew periods
Septoria leaf spot Septoria lycopersici Optimum temperature 20-25°C + high relative humidity and long dew periods
Anthracnose fruit rot Colletotrichum coccodes

Other Colletotrichum species

Optimum temperature 20-24°C + free moisture
Late blight Phytophthora infestans Optimum temperature 18-24°C + high relative humidity, rainy weather, fog, or long dew periods

 

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