VCR – Vegetable Crop Report – August 27th

Crop Updates

Brassica Crops – Conditions continue to be favourable for Alternaria, downy mildew and white mould. As canopies fill in, some insects, like cabbage aphids are becoming harder to manage. If cross-striped cabbageworm or zebra caterpillars are found, do not include them in your main lepidopteran threshold counts that include cabbage looper, diamondback moth and imported cabbageworm. (Figure 1-2). Both zebra and cross-striped cabbageworm caterpillars tend to feed in groups and are found in one or two spots in the field and are unlikely to be found throughout the entire field. Zebra caterpillars are black with yellow strips and white spots while the cross-striped cabbageworm have black and white stripes across the body with yellow undersides.

**Figure 1** – Cross striped cabbageworm feeding on a Brussels sprouts plant.

**Figure 2** – Cluster of zebra caterpillars found on a garlic leaf.

Carrots – Monitor for red leaves as the first sign of aster yellows (Figure 1). Infected plants will have excessive shoot and root growth (Figure 2). Leafhopper numbers have been quite high this season but infectivity numbers have been low.
Now that most row canopies are now closed, look for signs of white mould developing between the rows (Figure 3)

Figure 1. Red tinged carrot leaves from aster yellows infection. Photo credit – Hannah Van Boekel

Figure 2. Excessive root and shoot growth from aster yellows infection. Photo credit – Hannah Van Boekel

Figure 3. Cottony white growth on dead/dying leaves in the bottom of the carrot canopy.

Cucurbits – Cucurbit downy mildew has been confirmed on August 27^th in commercial cucumbers in Elgin County. This is the first report in Ontario for 2026. The first report is much later than previous years, similar to other regions in the northeastern US. Cucumber and cantaloupe growers should be on a downy mildew targeted fungicide program for the rest of the season.

Symptoms of downy mildew on cucumber leaves.

Garlic – If you are considering purchasing new planting stock for the 2026-2027 field season, remember that low levels of stem and bulb nematodes can be found in planting stock. Testing one bulb for every 200 lbs of planting stock or one in every 1000 bulbs could be a good indication of if there are nematodes present. Along with avoiding stem and bulb nematode, here are some other things to consider before purchasing new planting stock: https://onvegetables.com/2022/08/05/garlicplanting/. Clean seed is still available for purchase from the SPUD Unit Clean Seed Program for the 2026-27 Season. Ontario growers who are members of the Garlic Growers Association of Ontario (GGAO) are now able to order clean seed (roundels) in multiples of 50 by E-mailing garlicgrowersofontario@gmail.com. Roundels take an average of 2-3 growing seasons to reach a marketable size. Field trials between 2021-2025 show an average of a 25% increase in yield if planting a similar size clove of clean seed compared to a conventionally grown clove. The full results of this trial and a summary of the benefits of clean seed can be found in this previous article published October 2025: https://onvegetables.com/2025/10/10/cleanseed/. Once planting stock is obtained, consider planting in September opposed to October. Results from the past 3 years of planting date trials indicate that the later garlic is planted, the greater the amount of risk there is to losing yields at harvest the following July. To determine if planting earlier would result in winterkill or have negative impacts on yield, a planting date trial was conducted in Dashwood from 2022 to 2025 with seven planting dates starting in August and continuing roughly every three weeks until December, along with a single planting date in March or April every spring. In all three years, earlier planting dates had better winter survival, even though they also had the most leaves emerged prior to snow cover / the ground freezing. In the first two years the earliest dates also had the highest yields but there was a slight decrease in yields if the cloves were planted as early as August 21st. The full results of this trial can also be found here: https://onvegetables.com/2026/08/18/when-is-the-best-time-to-plant-garlic/

Onions – Harvest continues. Royal MH can be applied for sprout control when 50% of the crop has lodged and there are green tops that are still present to absorb the product. Apply at least 10 days before the crop is harvested. The 2026 Muck Crops Research Station Grower Field Day is September 3rd from 8:30 AM– 4:00 PM at the Muck Crops Research Station (Ontario Crops Research Centre – Bradford). Check out cultivar evaluations and research trials at 1125 Woodchoppers Lane, King, ON.

Potatoes – Late blight has recently been reported in Michigan. With cooler nights and frequent rains, the risk of disease and specifically late blight has increased (Figure 1). Ensure later maturing crops remain protected with late blight specific fungicides. Early blight and grey mould have also been increasing in fields.
Potato leafhopper numbers have been higher than usual this past season. Monitor for signs of hopperburn. Leafhopper nymphs can sometimes be seen on the underside of potato leaves (Figure 2).

Figure 2. Potato leafhopper nymph on the underside of potato leaf

Tomatoes – Late blight symptoms have been found in a backyard garden on tomatoes in Eastern Ontario (Lanark County). No late blight has been found in commercial fields but risk remains high with recent weather.

Figure 1. Late blight symptoms on tomatoes in a backyard garden.

Pest Degree Day Forecasting

County	Carrot Rust Fly	Onion Maggot	Carrot Weevil	Aster Leafhopper	Tarnished Plant Bug	Cabbage Maggot	Seedcorn Maggot	European Corn Borer
THRESHOLD	329-395, 1399-1711	210-700, 1025-1515	138-156, 455+	128+	40+	314-398, 847-960, 1446-1604	200-350, 600-750, 1000-1150	See legend below
Bruce***	1652	1513	1121	884	578	1246	1513	776
Essex*	2262	2106	1656	1372	980	1803	2106	1236
Chatham-Kent*	2097	1942	1500	1228	864	1645	1942	1103
Norfolk**	1951	1801	1384	1126	768	1517	1801	1002
Huron***	1808	1662	1252	1002	666	1384	1662	885
Wellington**	1749	1609	1217	973	642	1342	1609	857
Simcoe County***	1794	1654	1265	1022	695	1391	1654	908
Durham***	1904	1761	1359	1110	776	1490	1761	995
Peterborough	1793	1652	1257	1013	677	1386	1652	897
Kemptville***	1901	1761	1357	1111	778	1488	1761	997
Sudbury***	1567	1444	1103	892	595	1214	1444	790
Timiskaming***	1532	1408	1074	864	566	1181	1408	763
Lambton**	2001	1849	1424	1170	814	1561	1849	1048
Thunder Bay	1420	1299	970	772	493	1074	1299	676
Middlesex*	2012	1863	1442	1184	826	1579	1863	1060
Renfrew	1644	1522	1174	966	687	1287	1522	871