Garden Pests and Diseases

By Maryssa Paulsen, Education Intern

Now that you’ve worked hard to gather your seeds and plant them, we want to help your garden succeed as you continue to watch and care for it. You may be concerned about pests or diseases or maybe you’ve already noticed signs of their presence. We hope to alleviate some of those concerns and encourage positive thoughts for your garden!

Common Pests

  • Japanese beetle: a very small, shiny metallic-green beetle with coppery-brown wings. Japanese beetles feed on over 350 species of plants, including foliage, flowers, fruits, vegetables, ornamentals, field crops, and weeds. Source:

    japanese beetle

    Japanese beetle

  • Boxelder bug: black in color with three red lines on the thorax and red lines on the wing margins. Boxelder bugs feed on leaves, flowers and seedpods of boxelders and silver maples, generally doing only minor damage to the trees. Source:
  • Aphids: soft-bodied sucking insects with a pair of cornicles (tailpipe-like structures) projecting from the rear end of their abdomen. Adult aphids may or may not be winged. These pests produce a sugary substance that may attract ants and support the growth of a fungus called sooty mold. There are over 4,000 species of aphids; the most serious in Wisconsin include the green peach, melon, potato and soybean aphids. Source:




With the humidity we’ve been experiencing, your garden or crops are susceptible to disease. A common type of disease you may have heard of is powdery mildew. Another sign of disease is the bacterial speck or spot, which affects your tomato plants.

  • Powdery mildew: a cosmetic, non-lethal disease that occurs on the above-ground parts of deciduous trees and shrubs, agricultural crops, ornamental plants, and indoor houseplants. As its name sounds, it has a powdery, white appearance, especially on leaves.

    Powdery mildew


  • Bacterial speck (small black dots) favors cool, moist conditions while bacterial spot (small yellow to brownish red spots) favors warm, moist conditions. Both diseases substantially reduce yield and marketability of your crop. Unfortunately, a plant with bacterial speck or spot cannot be cured. Prevention methods for both diseases are similar. Source:

We hope you have found this piece helpful and wish your garden much success this summer. Should you have any questions, do not hesitate to contact the UW-Extension Horticulture Help Desk.