VCR – Vegetable Crop Report – August 13th

Crop Updates

Brassica Crops – This is likely the last week for transplanting late broccoli in southern regions. Alternaria pressure has been high in many fields since there has been ideal environmental conditions for disease development. For a list full list of registered products on Alternaria for broccoli, Brussels sprouts, cabbage, cauliflower, and kale go to the Ontario Crop Protection Hub. Clubroot symptoms are pronounced in fields that have had excess moisture. In cabbage, fields with a history of Sclerotinia white mould are showing disease development and some fields also have bacterial soft rot. Keep on the lookout for downy mildew symptoms, especially in the headland where the crop may be shaded or are protected by winds drying out leaves (Figure 1). Diamondback moths, imported cabbageworms, tarnished plant bugs and aphids continue to remain elevated in many areas. Incorporate crop debris as soon as possible to limit the amount of inoculum and pest development that can spill over to later plantings.

**Figure 1** – Downy mildew lesions on the lower leaves of a broccoli plant.

**Figure 2** – Late season flea beetle feeding – April 11, 2026.

Cucumbers – Downy mildew was reported in Southern Michigan and Northern Ohio earlier this week. We still haven’t seen it here in Ontario, but now that it is in the Great Lakes region, growers should definitely maintain their fungicide programs. We are seeing some other foliar diseases in the Ontario crop, including powdery mildew. Powdery mildew will not be controlled by the standard downy mildew preventative program. Chlorothalonil will provide a basic level of protection against powdery. As with most foliar diseases, the best controls are preventative. There are several fungicides with more targeted powdery mildew activity, however they will not provide downy control.

Spider mites continue to be a problem in both Kent and Elgin/Norfolk. In-harvest control options are limited as most products have a longer pre-harvest interval. Frequent scouting can help identify hot-spots before they spread, offering the option of spot treatments or border sprays that can be worked around the harvest interval requirements.

Garlic – With garlic in storage, plans for next years crop are underway. If you are purchasing planting stock for this fall, remember that low levels of stem and bulb nematodes can be found in planting stock that otherwise looks great. It is a good idea to conduct stem and bulb nematode testing before purchasing planting stock from another farm. 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. Here’s an article summarizing the things to consider before purchasing new garlic planting stock: https://onvegetables.com/2022/08/05/garlicplanting/. Clean seed is 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/.

**Figure 3** – A visual comparison of garlic with identical genetics that has been through the Clean Seed Program (CSP) at the SPUD Unit compared to a conventional bulb. The ‘Polish White’ bulbs pictured were harvested from the same field in July 2025. Both bulbs were grown under the same conditions for the past two growing seasons and had the same average starting clove weight in Fall 2024. The bulb from the CSP (right) was originally grown from a roundel that was planted in fall 2022.

Onion – Transplants continue to be harvested, and direct seeded onions are starting to lodge. With cool mornings, fog, dews and extended periods of leaf wetness, conditions have been favourable for onion downy mildew in many regions (Figure 4). A full list of products registered for onion downy mildew can be found on the Ontario Crop Protection Hub. Bacterial rots are common on plants that are in lower areas of the field that received flooding over the past month (Figure 5). Stemphylium leaf blight and tipburn continues to progress. 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. Now is a great time to assess damage plots that were put up in the spring. Determining the number of plants that survived out of the 100 germinated/transplanted plants that were counted at the start of the season is valuable information to determine what percentage of plants died due to onion maggot, drought, or some other pest.

**Figure 4 –** Heavy dews can be ideal conditions for downy mildew development.

**Figure 5 –** Lower leaves turning yellow and some onion plants with symptoms of bacterial rot in a low area of a field.

Pumpkins and Squash – Mid-to-late August is prime powdery mildew time. There are a lot of good fungicide products to choose from, at a wide range of price points. When selecting products, keep in mind that there is widespread powdery mildew resistance to the group 11 fungicides and some of the group 3 products as well. Always rotate between the chemical families to prevent the development of new resistant strains.

In areas where spider mites have been identified in cucumbers, keep a close eye out on the other cucurbit crops. Damage can be hard to spot before they have caused substantial stress to the crop. They often build up at the field edges, especially close to gravel roads or dusty laneways, however hot spots can occur in the field centers as well.

Sweet Corn – Corn earworm moths are now on the move. Moth catches in the mid-west are starting to increase, which is a sign that we will also begin to see increased pressure here in Ontario. Moth activity tends to be very spotty and often tracks with severe storms. Protect plants as soon as the green silks appear.

Aphid populations have also been high in some sweet corn fields this week. Aphid activity pre-tassel can impact the kernel set. Developing populations on the ears will impact marketability. We often see good populations of aphid predators in sweet corn fields which may help maintain clean plants under low aphid pressure. Keep in mind that populations can explode quite quickly. Especially under hot, dry conditions.

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***	1456	1331	977	767	500	1090	1331	671
Essex*	2027	1884	1473	1215	862	1607	1884	1092
Chatham-Kent*	1880	1738	1335	1088	763	1466	1738	976
Norfolk**	1741	1604	1227	995	675	1347	1604	883
Huron***	1611	1478	1106	882	586	1226	1478	778
Wellington**	1558	1431	1078	860	568	1190	1431	757
Simcoe County***	1602	1475	1125	908	620	1238	1475	807
Durham***	1690	1560	1198	975	679	1315	1560	872
Peterborough	1601	1473	1117	899	601	1232	1473	796
Kemptville***	1697	1570	1204	985	690	1322	1570	883
Sudbury***	1382	1272	970	785	527	1068	1272	696
Timiskaming***	1352	1241	946	763	504	1040	1241	674
Lambton**	1785	1646	1260	1032	716	1384	1646	923
Thunder Bay	1252	1144	854	682	441	944	1144	599
Middlesex*	1796	1660	1278	1046	727	1401	1660	935
Renfrew	1465	1354	1038	852	606	1140	1354	768