Something has been at your plants. Maybe it happened overnight. Maybe it crept up gradually over two weeks until you could no longer ignore it. Holes in leaves. White powder on the stems. Yellow patches spreading across what used to be healthy foliage. Fruit rotting before it ripens. Whole branches wilting on a plant that was fine last week.
Late summer is when garden problems peak in Ontario. The combination of heat, humidity, stressed plants that have been producing for months, and the seasonal population explosion of insects and fungal spores creates the perfect conditions for pests and disease to take hold quickly and spread fast. August and September are the months when the most gardening distress calls happen, and almost always for the same handful of problems.
The good news is that most late summer garden problems are identifiable, treatable, and in many cases preventable if you know what you are looking at. Here is a comprehensive guide to the most common pests and diseases affecting Zone 6a Ontario gardens right now, how to identify each one accurately, and what to do about it.
Before You Treat Anything: How to Diagnose the Problem Correctly
The single most common mistake gardeners make when they see something wrong with their plants is reaching for a treatment before they have correctly identified the problem. Applying a fungicide to a pest problem does nothing. Applying an insecticide to a nutrient deficiency makes it worse. Taking thirty seconds to correctly identify what you are dealing with before you act saves time, money, and often the plant itself.
Ask these four questions before doing anything:
Is the damage on the leaves, stems, roots, or fruit? Location tells you a great deal about the likely cause. Leaf damage is most commonly pests or fungal disease. Stem damage often indicates borers, rot, or physical damage. Root damage points to soil-dwelling pests or root rot from overwatering. Fruit damage can be pests, disease, or environmental stress like blossom end rot.
Is there a visible insect, egg cluster, or webbing present? If yes, you are dealing with a pest. If no visible organism is present, the cause is more likely a disease, environmental stress, or a nutrient deficiency.
Is the damage spreading? A spreading problem is more urgent than a stable one. Fungal diseases and certain pests spread rapidly across a plant and to neighbouring plants and need faster intervention than a localised issue.
Did the problem appear after a rain or a period of high humidity? Fungal diseases are almost always triggered or worsened by wet, humid conditions. A problem that appeared or worsened after a stretch of Ontario August humidity is most likely fungal in origin.
Common Late Summer Pests in Ontario Gardens
Aphids
What to look for:
Aphids are tiny, soft-bodied insects ranging from 1 to 3 millimetres long, found in clusters on the undersides of leaves, on new growth tips, and on flower buds. They come in green, black, yellow, and grey varieties depending on species and host plant. Look for distorted, curled, or yellowing new leaves, sticky residue on leaves and stems called honeydew, and a black sooty mould that grows on that honeydew. Aphid populations can explode from a handful to thousands on a single plant within days in warm August conditions.
What to do:
Start with the simplest intervention first. A strong stream of water from a garden hose directed at infested stems and leaf undersides physically removes most aphids and is often sufficient for light to moderate infestations. Repeat every two to three days for two weeks.
For more established infestations, insecticidal soap spray applied directly to aphids on contact is highly effective and breaks down quickly without harming beneficial insects once dry. Neem oil spray is another organic option that disrupts the aphid life cycle when applied consistently every five to seven days.
Encourage natural predators by planting or protecting habitat for ladybugs, lacewings, and parasitic wasps, all of which feed voraciously on aphids and can bring a population crash naturally within one to two weeks without any intervention.
Do not treat if:
You see small, mummified brown aphids in the colony. These indicate parasitic wasps have already been at work and the infestation is already in natural decline.
Tomato Hornworm
What to look for:
Tomato hornworms are among the most dramatic garden pests you will encounter in an Ontario garden. They are large caterpillars, up to 10 centimetres long, bright green with white diagonal stripes and a distinctive horn at the tail end, that feed on tomato, pepper, and eggplant foliage. The most reliable sign of their presence is not the caterpillar itself, which is remarkably well camouflaged, but the damage: large sections of leaf stripped completely from stems, leaving behind bare stems and dark green droppings the size of small pellets on leaves below.
Look carefully along defoliated stems and on the undersides of leaves near the damage. Hornworms rest during the day and are most active in the evening.
What to do:
Hand picking is the most effective and immediate treatment for hornworms. Drop them into a bucket of soapy water or relocate them far from your garden. Check plants every two days since new caterpillars can appear as eggs hatch through late summer.
Critically important: If you find a hornworm covered in small white rice-like structures along its back, do not remove it. Those are the cocoons of parasitic braconid wasps whose larvae have already killed the caterpillar from inside. Leaving that hornworm in place allows a new generation of beneficial wasps to emerge and continue controlling hornworm populations in your garden naturally.
Squash Vine Borer
What to look for:
Squash vine borers are one of the most devastating late summer pests for Ontario gardeners growing zucchini, squash, and pumpkins. The adult is a day-flying moth that lays eggs at the base of squash stems in July and August. The larvae bore into the stem and feed from the inside, causing sudden and dramatic wilting of entire vines that were healthy the day before. Cut an affected stem lengthwise and you will find the cream-coloured larvae inside along with masses of greenish frass.
The first sign is usually a vine wilting despite moist soil. Check the base of the stem for small holes with sawdust-like frass around them.
What to do:
Once larvae are inside the stem, treatment options are limited. You can attempt to cut the stem lengthwise, remove larvae manually, and bury the damaged stem section under moist soil to encourage new root formation above the damage. This works sometimes but not always.
Prevention is far more effective than treatment for squash vine borers. Wrapping the base of squash stems with aluminium foil at planting time creates a physical barrier to egg laying. Row cover placed over young squash plants and removed only for pollination prevents adult moths from reaching the stems at all during their egg-laying period in July.
Spider Mites
What to look for:
Spider mites are not visible to the naked eye individually but their damage and webs are unmistakable. Look for fine, stippled yellowing or bronzing of leaves that appears first on the upper surface, along with fine silky webbing on the undersides of leaves and between stems. Spider mites thrive in hot, dry conditions and populations explode during Ontario's hottest August stretches, particularly on plants that have been water-stressed.
To confirm spider mites, hold a white piece of paper under a suspect leaf and tap the leaf firmly. Tiny moving specks on the paper confirm mite presence.
What to do:
Increase watering and humidity around affected plants immediately since spider mites hate moisture. Spray plants thoroughly with water focusing on leaf undersides to physically disrupt mite populations. Insecticidal soap or neem oil applied to leaf undersides every five to seven days is effective against mites. Avoid broad-spectrum insecticides since these kill the natural predators that keep spider mite populations in check and often make infestations significantly worse.
Cabbage Worms and Cabbage Loopers
What to look for:
If you are growing fall brassicas, including broccoli, cabbage, kale, or cauliflower, cabbage worms and cabbage loopers are the pests most likely to find them. Cabbage worms are velvety green caterpillars about 3 centimetres long that blend almost perfectly with brassica foliage. Cabbage loopers are similar in colour but move with a distinctive looping motion. Both leave irregular holes in leaves and small dark pellet-like droppings on foliage.
Look carefully on both the upper and lower leaf surfaces of all brassica plants every few days through August and September since populations build quickly as fall brassica plantings establish.
What to do:
Hand-picking is effective for light infestations. For larger populations, Bacillus thuringiensis, commonly sold as BT, is a naturally occurring soil bacterium that is lethal to caterpillars when they consume treated foliage but completely harmless to birds, bees, beneficial insects, and humans. It is the most effective and ecologically responsible treatment for cabbage worms and loopers available to home gardeners and is available at garden centres. Apply in the evening when caterpillars are most active and reapply after rain.
Row cover placed over fall brassica transplants immediately after planting prevents adult butterflies and moths from laying eggs on plants in the first place and is the most reliable preventative strategy available.
Common Late Summer Diseases in Ontario Gardens
Powdery Mildew
What to look for:
Powdery mildew is the most common fungal disease in Ontario gardens and one of the easiest to identify. It appears as a white or grey powdery coating on the upper surface of leaves, starting as small circular patches that spread to cover entire leaves, stems, and flower buds as the disease progresses. It affects a wide range of plants including zucchini, cucumbers, pumpkins, roses, bee balm, phlox, and many ornamental perennials.
Powdery mildew is triggered by warm days combined with cool, humid nights, exactly the conditions that characterise August and September in Ontario. Unlike most fungal diseases it does not require wet leaf surfaces to spread and actually thrives in dry conditions with high ambient humidity.
What to do:
Remove and dispose of heavily affected leaves immediately, placing them in the garbage rather than the compost since powdery mildew spores survive composting in most home compost systems. Improve air circulation around affected plants by removing crowded foliage and increasing spacing where possible.
For treatment, a spray of one part milk to nine parts water applied to affected foliage every week has genuine research support as an effective organic powdery mildew treatment. Baking soda solution (5 grams baking soda dissolved in 1 litre of water with a few drops of liquid dish soap) is another accessible home remedy that changes the leaf surface pH and inhibits fungal growth. Neem oil and potassium bicarbonate-based organic fungicides available at garden centres are more powerful options for serious infestations.
Important: Powdery mildew at the end of the season on mature zucchini or cucumber plants that have already produced abundantly is largely cosmetic and does not require treatment. Reserve your intervention energy for valuable plants with significant production still ahead of them.
Tomato Blight (Early and Late)
What to look for:
Tomato blight is the most feared late summer disease for Ontario vegetable gardeners and for good reason. It spreads rapidly in the warm, humid conditions of August and can devastate a tomato planting in a matter of days if conditions are right.
Early blight appears as dark brown spots with concentric rings, often described as a target or bullseye pattern, beginning on the lower, older leaves of the plant and working upward. Yellowing surrounds the brown spots. Early blight is common and serious but generally slower moving than late blight.
Late blight is more aggressive and more destructive. It appears as dark, water-soaked patches on leaves that rapidly turn brown and papery with a pale green to yellow border. In humid conditions a white fuzzy growth may be visible on the undersides of affected leaves. Late blight spreads with frightening speed and can kill an entire tomato plant within days of first appearing. It also affects potato plants and spreads between them.
What to do:
At the first sign of either blight, remove all affected foliage immediately and dispose of it in the garbage, never the compost. Avoid working with plants when foliage is wet since this spreads spores on your hands and tools. Water at the base of plants only and never overhead. Stake tomatoes to improve air circulation and keep foliage off the ground.
For early blight, copper-based organic fungicides applied preventatively or at the first sign of disease slow its spread effectively. For late blight, once it is established in a plant it is very difficult to stop completely. Remove affected plants entirely if late blight is severe and the season is nearly over to prevent spread to neighbouring plants and potato crops.
The single most effective blight prevention strategy is not allowing tomato foliage to stay wet. Water in the morning, mulch heavily to prevent soil splash onto lower leaves, and remove the bottom 30 centimetres of foliage from tomato plants to create an air gap between the soil and the plant.
Blossom End Rot
What is it:
Blossom end rot is not a disease caused by a pathogen. It is a physiological disorder caused by calcium deficiency in developing fruit, most commonly seen in tomatoes, peppers, and zucchini. It appears as a dark, sunken, leathery patch at the blossom end (bottom) of the fruit, often starting small and expanding to cover a third or more of the fruit surface. Affected fruits are not salvageable.
What causes it:
Calcium is not absorbed efficiently by plants when soil moisture fluctuates dramatically between wet and dry. Inconsistent watering is the primary cause of blossom end rot in Ontario gardens, not a lack of calcium in the soil. Periods of drought stress followed by heavy watering, which is common during Ontario's August heat, trigger the condition repeatedly.
What to do:
Consistent, deep watering is the most important fix. Mulch heavily around affected plants to retain soil moisture and reduce the wet and dry cycles that trigger calcium absorption problems. Remove all affected fruits immediately since they will not recover and leaving them on the plant slows production of new healthy fruit.
Calcium sprays applied to foliage can help in the short term but do not address the underlying moisture consistency issue. Fix the watering first and blossom end rot almost always resolves itself within one to two weeks.
Cucumber Mosaic Virus
What to look for:
Cucumber mosaic virus affects cucumbers, zucchini, squash, and many other vegetables and ornamentals. It appears as a mottled yellow and green mosaic pattern on leaves, distorted or stunted new growth, and in cucumbers, fruit that is pale, mottled, and bitter. It is spread by aphids moving between infected and healthy plants.
What to do:
There is no cure for plants infected with cucumber mosaic virus. Remove and dispose of infected plants immediately to prevent aphid vectors from spreading the virus to healthy plants. Control aphid populations aggressively on all nearby plants using the methods described earlier. Do not compost infected plant material.
The Most Important Late Summer Disease Prevention Habits
Clean tools between plants using a diluted bleach solution or rubbing alcohol to prevent spreading fungal spores and viral particles from plant to plant as you work through the garden.
Remove fallen fruit and plant debris from the base of plants immediately. Fallen fruit harbours pests and disease and creates conditions for soil splash that spreads fungal spores onto lower foliage.
Never compost diseased plant material. Home compost piles rarely reach the temperatures needed to reliably kill fungal spores, viral particles, and pest eggs. Always bag diseased material for the green bin or garbage.
Rotate crops next season. Many soil-dwelling fungal diseases and pests overwinter in the soil and re-infect the same crops the following year. Moving crop families to different bed locations each season breaks this cycle and is the single most effective long-term disease prevention strategy available.
When to Ask for Help
Most of the problems covered in this guide are manageable with the organic treatments described here. But some situations call for more targeted intervention or expert identification, particularly when a problem is spreading rapidly, affecting multiple plant species at once, or when you genuinely cannot identify what you are looking at.
Stop by Lakeside Garden Gallery and bring a photo or a sample of the affected plant material. Our team can help you identify the problem accurately and recommend the right organic treatment for your specific situation. Getting the identification right is always the most important first step. 🌿🔍