Aphids
One of the most common garden pests are aphids, small sap-sucking insects that love to dine on new growth. They come in a variety of colors (often yellow, green or gray) and can quickly cover plants like a bumpy armor.
The defense, Kranz and Savio said, is to check your plants daily and at the first sighting, spray the aphids off the plants using a hose. Once removed, they don’t return, Savio said, but you have to act quickly because aphids multiply rapidly. Aphids are particularly fond of new growth on roses and brassicas (the cruciferous vegetables), such as broccoli and kale.
Powdery mildew
This fungal disease is partial to the cucurbit family (melon, cucumbers and squash) and often appears late in the growing season as white powdery spots on the top of leaves. Eventually the leaves will look as though they’re dusted in flour, then will turn yellow and dry out.
To prevent the disease, look for disease-resistant seedlings and avoid overhead watering and watering late in the day because the water, coupled with the cool night air, encourages mildew.
If you spot the fungus act quickly by cutting off the infected foliage and discarding it, but you can’t cut off too much without harming the plant. Kranz has used neem oil; others find a milk spray of half milk and half water to be helpful, but Kranz said she hasn’t found anything to be completely effective.
Spider mites
Mites, about the size of a period, thrive in hot, dry conditions, sucking the fluids from stressed plants. Mites are hard to spot, so the first evidence of an infestation usually is mottled, spotty leaves, followed by curled, dried-out leaves and a fine webbing around the plant.
The infestations can wax and wane, Savio said. Last year, spider mites struck the tomatoes in her well-watered Pasadena garden. When you first see them, you can try washing them off your plant with a blast from the hose, but Savio is merciless when it comes to this pest. She’ll cut off an infected branch, making sure to put it in a plastic bag and dumping it in the trash and pull out the entire plant if the pests return.
“I will not tolerate them at all in my garden,” she said. “I’ll make one or two attempts (to combat them), but when I see that fine webbing, even if the plant is full of fruit, I take it out completely.”