Managing Pests and Diseases

One of the unfortunate realities of growing food is that there is a multitude of diseases and pests which can devastate your garden if left unchecked. To protect the results of your hard work, you will need to take proactive steps to prevent and treat any problems as soon as they arise.  
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As your plants grow, it’s important to check them often for signs of damage. Each pest or infection leaves visual clues which will help you identify the source of the problem. Prevention and early intervention are key to saving a plant that is under attack! Let’s explore some common culprits of plant damage and what you can do to combat them.  ​

If your plants have entire leaves or branches missing and it looks like they were cleanly cut off, an animal such as a rabbit or deer is likely to blame. Rabbits typically avoid tomatoes and onions, but everything else is fair game. They especially love cabbages, broccoli, and carrots. Deer will eat just about anything. The most reliable way to protect your garden from animals is to construct a physical barrier. This can be a cloche, a wire fence, or an inexpensive floating row cover made out of garden fabric or durable plastic netting. I do not recommend using thin bird netting - rabbits can and will bite through it. 

A tomato plant damaged by deer. Image by Spacemagic (unmodified).

I have found that you can construct a low fence against rabbits by stacking repurposed wooden boards on top of one another and using stakes driven into the earth on either side to support them (see diagram below). ​​Rabbit fences should be at least 2 feet tall. 

A simple and inexpensive DIY fence

If you notice smaller round bites along the edges of your leaves, slugs or caterpillars are probably causing the damage. Slugs usually feed on plants at night and then retreat to a cool, damp hiding place, while caterpillars often live on the plant. One easy way to identify caterpillars is that they often leave silvery webbing across the leaves.​

Broccoli plants damaged by slugs. Image by Downtowngal (rotated).

If the leaves are peppered with tiny round holes, they are likely being eaten by flea beetles. 

Potato leaves damaged by flea beetles. Image by Maine Department of Agriculture.

To stop an infestation and keep these insects away from your plants in the future, you may need to use a chemical insecticide. Many gardeners swear by a product called Sevin to do this. Sevin comes in spray or powder form and is extremely effective because it contains very powerful chemicals. If you need to use it, I recommend a light dusting of the powder because it stays on the leaves longer than the spray. You will need to reapply the product after each rain. 

​While these chemicals are unlikely to end up in your food, if you do not want to use chemical products, you can try some natural remedies. The best way to treat a plant damaged by caterpillars is to remove them by hand. Check the tops and undersides of leaves for eggs and caterpillars – they may be the exact same color as the leaf, so look carefully! If you find live caterpillars or eggs,  kill or relocate them far from your garden. 

​To prevent slugs from roaming your garden, try to eliminate any places where they could shelter. Clear debris such as branches, piles of leaves, and wooden boards they could hide under.  To keep them off of your plants, sprinkle a barrier of food grade diatomaceous earth all the way around the stem of the plant. Diatomaceous earth is a completely natural and safe substance made of powdered silica rock. Slugs will not crawl across it. However, you need to reapply diatomaceous earth after every rain because it becomes ineffective when wet. For this reason, it's more cost efficient to purchase a large bag of it. 

If treating flea beetles, you can sprinkle a light coating of diatomaceous earth on the leaves of the plant to repel the insects. Diatomaceous earth kills beetles and other insects by drying out their shells. If your plant is getting big and has many leaves, and only a few leaves are damaged by flea beetles, you can just leave the plant alone. If the flea beetle infestation is mild, the plant will recover from and outgrow the damage on its own. 

If your leaves are covered in what looks like white powder, the plant is likely suffering from a fungal infection called powdery mildew. Other fungal infections can show up as black or yellow splotches on leaves. The best way to prevent these types of infections is to ensure good air circulation around your plants by properly spacing them apart and to water your plants from the bottom so their leaves do not get wet. To treat powdery mildew and some fungal infections, you can spray the leaves with neem oil or a copper-based fungicide, both of which are natural. However, for other fungal infections like tomato blight, it's best to completely remove the infected plant so that the fungus doesn't spread to other plants. 

Powdery mildew on an eggplant. Image by Scot Nelson (unmodified).

Of course, there are dozens of other pests and maladies which can affect your garden. If you are not sure what is damaging your plants, I recommend calling your local agricultural extension office. Extensions are usually partnerships between the USDA and local universities, and their job is to provide agricultural education to the public. They often operate phone lines which gardeners can call with questions. They will help you identify the source of the problem and provide advice for treating it. You can look up your local extension office here. 
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As always, if you have any questions or tips to share, please comment below! ​