Leaf Damage

Leaves are critical to plant health.
 
Healthy leaves perform photosynthesis. This is where plants make sugar from sunlight.
 
Healthy leaves also help plants get rid of excess water in a process called guttation. And leaves make shade, preventing sunburn (ours and theirs). But leaves are easily damaged.

​We prune them, bump into them, and neglect them. But that’s not all.
 
Leaves are damaged from the inside by disease and malnutrition, and they can be chewed on, sucked dry, or beaten senseless by external forces.
 
Note: When you spot leaf damage, it is a good idea to isolate the plant, if possible. Then, as you read through this information, use the links for more detailed information on each condition, disease, pest, and effective treatments in each case.
 
Recognizing the source of the problem will help you find a solution. Identify the insects you see so that you will know if they are helpful or harmful. And decide on your tolerance level for plant disease and other garden problems. Keeping your plants healthy goes a long way toward reducing your workload and improving your harvests.
 
Let’s start with diseases.
 
Diseases that cause leaf damage
 
Leaf problems are often a gardener’s first sign of disease. These diseases can be bacterial, fungal, or viral, and each has unique characteristics that can help you identify and treat the problem.
 
Bacterial diseases often start as water-soaked lesions on leaves that dry out and fall away, creating shothole patterns. Those lesions commonly have yellow halos. Wilting is another common symptom of bacterial disease.
 
Some common bacterial diseases that cause leaf damage include the following:

• Angular leafspot
• Common bacterial blight
• Halo blight

Fungal diseases often appear as angular lesions or bullseye-shaped spots on leaves that spread quickly. These spots have clearly defined edges. Fungal diseases also appear as tiny black dots, and you may be able to see mycelium threads. Affected leaves may become thick or rubbery. Some fungal diseases that cause leaf damage in your garden include the following:

• Alternaria leaf blight
• Early blight
• Fusarium wilt
• Gummy stem blight
• Pea seed-borne mosaic
• Peach leaf curl
• Powdery mildew
• Root rots
• Rust
• Septoria leaf spot
• Shot hole
• Sooty mold
• Verticillium wilt

Early symptoms of peach leaf curl

Orange rust on rose leaves

Viral diseases block the flow of fluids inside our plants. Viral diseases appear as irregular patches of mottling or mosaic patterns without clearly defined edges or yellow streaking. Infected leaves often curl or twist. Affected leaves may develop purple veins. Wilting can also occur. Viral diseases tend to progress slowly. Viral diseases that cause leaf damage include the following:

• Bean common mosaic
• Beet western yellows
• Calico
• Curly top
• Cucurbit yellow stunting disorder
• Grapevine fanleaf
• Pepper mottle
• Potato mop top
• Potato viruses A, X, and Y
• Squash mosaic
• Strawberry vein banding
• Tomato and tobacco mosaics
• Tomato bushy stunt
• Tomato ringspot
• Tomato spotted wilt
• Tomato yellow leaf curl
• Zucchini yellow mosaic​

Zucchini leaf infected with yellow mosaic

​Another group of leaf-damaging pests is called oomycetes.  These are algae-like creatures that damage leaves with downy mildew and potato blight.

Where do leaf-damaging diseases come from?
 
Leaf-damaging plant diseases can be carried by a variety of pests, including aphids, beetles, flies, gnats, leafhoppers, moths, nematodes, psyllids, thrips, weevils, and whiteflies are common disease vectors. Diseases that cause leaf damage can also be carried on neighboring plants, old cigarette butts, our shoes, garden tools, and the wind.
 
How do you know what the problem is?
 
Some diseases are easy to identify. The red, rubbery, wrinkled leaves of peach leaf curl are unmistakable. But other causes share symptoms. For example, mosaics usually indicate a viral disease but can also mean your plants are suffering from chemical overspray. You will have to take other factors into consideration to sort things out. If you need more detailed information, check out my book What’s Growing Wrong? Or contact me, and we can sort it out together.
 
Leaf damage caused by suckers
 
Many pests damage our plant leaves by sucking out sap. This is most often done by invertebrates. Invertebrates do not have backbones. Being spineless in this case does not imply that these creatures lack courage. On the contrary, they tend to be highly flexible, adaptable, and potentially destructive to your garden. This group includes insects, sowbugs, snails, spiders, and more. Did you know that 97% of all animal species are invertebrates? I didn’t either.

Common sucking pests include the following:

• Aphids
• Leafhoppers
• Mealybugs
• Mites
• Pillbugs
• Scale insects
• Sharpshooters
• Stinkbugs
• Thrips
• Whiteflies

Mealybugs on citrus leaf

Spider mites on horseradish leaf

Leaf damage caused by chewers

You can usually tell when something has been chewing on your garden plants. If you see ragged leaf edges, it’s probably herbivore feeding, while smoother edges and skeletonization usually indicate insect and other invertebrate feeding. Common leaf chewing pests include the following:

• Beetles
• Caterpillars
• Crickets and grasshoppers
• Earwigs
• Katydids
• Leafminers
• Slugs and snails

Leafminer damage on beet leaf

Snail feeding damage on Gerbera daisy plant

Chipmunks, deer, groundhogs, livestock, mice, rabbits, rats, squirrels, and occasionally voles will also cause leaf damage.
 
Nutrient disorders that cause leaf damage
 
Healthy leaves are deep green, to one degree or another. It depends on the plant species.
When things start going wrong, they may turn into paler versions of themselves. This yellowing, or chlorosis, can be due to nitrogen or potassium deficiencies.
 
Other symptoms (and causes) of nutrient-based leaf damage include the following:

• Burnt or dead leaf edges may mean there is too much salt in the soil for that plant.
• Purple-ish older leaves may indicate a phosphorus deficiency (or low soil temperatures).
• Yellowing alongside green veins (interveinal chlorosis) may indicate an iron or magnesium deficiency. It may also mean poor drainage or high soil pH.
• Yellow leaf edges, combined with weak stems, indicate your plants may need more potassium.

Iron deficient plum leaf

Nitrogen deficient melon leaf

The best way to prevent and monitor for nutrient disorders is with a lab-based soil test every 3-5 years. Don’t let the word “lab” scare you off. These tests are inexpensive and incredibly helpful. Do it once, and you’ll see what I mean!
 
Other causes of leaf damage
 
Leaf damage isn’t limited to people, pests, and diseases. There are environmental factors to consider. Sometimes you can fix these problems, and sometimes you can’t. The most common environmental factors that cause leaf damage include the following:

• Air pollution
• Chemical overspray
• Drought
• Overwatering
• Sunburn
• Wind damage

When diagnosing leaf damage, start with the specific type of plant. What species is it? Then consider its developmental stage. Is it a seedling, flowering, fruiting, or preparing for winter? Symptoms will appear differently as the year progresses and on different plant species. And this is why it is so important to monitor your plants.
 
I like to consider myself something of a detective when it comes to sorting out leaf damage and other garden issues. Good gardeners are always on the lookout for clues on the health of their plants. And leaves are nearly always the best place to start looking.