Tar spot: field report

1. DETECTION

Since 2021, the first signs of tar spot stroma (black spots) have been observed during the first two weeks of July in southwestern Ontario. The warm, humid conditions (relative humidity < 75 per cent) were conducive to tar spot infection throughout the growing season. In addition to the traditional tar spot areas along the north shore of Lake Erie (from Essex to Niagara), the disease was detected earlier in Middlesex, Oxford, and Perth counties, extending west toward Toronto (see Figure 1).

2. SPREADING

For the first time, tar spot was identified in central Ontario later in the season (September/October) at low levels. While the timing and low disease levels are unlikely to impact yield significantly, growers should remain vigilant and prepare for potential establishment in the region. This spread aligns with increased tar spot occurrences in New York, Pennsylvania, and, for the first time, Quebec (see Figure 1).

3. DISEASE DEVELOPMENT

The development of tar spot symptoms can vary significantly based on environmental conditions, particularly humidity and temperature. It typically takes two to four weeks for symptoms to appear from spore release from infected corn debris. New stromata can form within seven to 21 days post-infection. Favourable conditions can lead to multiple spore-release events and infection cycles throughout the growing season. In severe cases, complete plant blighting can occur within weeks. Scouting, disease forecasting models (like Sporecaster), and real-time disease monitoring are essential for identifying field risk.

4. WEATHER DRIVES THE DISEASE

Tar spot thrives in moderate temperatures with relative humidity above 75 per cent and at least seven hours of moisture on corn leaves. Early development and higher severity increase the likelihood of multiple disease cycles, potentially leading to plant shutdown under stress, such as moisture stress. This can negatively impact yield and test weight if the corn cannot reach physiological maturity, reducing grain fill.

5. DIFFERENCES IN HYBRIDS

Most commercial hybrids are susceptible to tar spot, though some exhibit greater tolerance; however, none are completely resistant. Grain Farmers of Ontario and Sustainable Canadian Agriculture Program- supported research is exploring potential novel sources of resistance through the Agriculture and Agri Food Canada Ottawa corn breeding program. Consult your local seed provider for tar spot hybrid ratings and other characteristics, such as standability and stalk rot tolerance.

6. FUNGICIDES

When it comes to fungicide applications, two key decisions for farmers are the choice of product and timing. In Canada, Veltyma DLX and Delaro Complete are registered for tar spot control in corn, while Miravis Neo offers suppression. Field performance will depend on disease severity, hybrid susceptibility, environmental conditions, application timing, and other factors.

7. FUNGICIDE APPLICATION TIMING

Research indicates that the most significant yield benefits arise from fungicide applications at the VT-R1 growth stages (tassel to silking). This timing is also most effective for managing Gibberella ear rot and DON mycotoxin. Applications made at R3 and later may show visual differences but have not consistently demonstrated a positive ROI based on Ontario and other research data. We look forward to seeing what the 2024 results reveal! Ensuring good coverage is crucial to maximizing fungicide efficacy.

8. OTHER DISEASES

When selecting hybrids and fungicides, consider other diseases such as Northern Corn Leaf Blight, common rust, gray leaf spot, and Gibberella ear rot. •