Leaf galls are abnormal plant growths caused by various parasites like insects, mites, fungi, bacteria, viruses, and nematodes. These galls appear as swellings on leaves and stems of plants and can range from barely noticeable bumps to large growths that severely distort the plant’s foliage. While mostly harmless, severe infestations of leaf galls can weaken and stress plants. Fortunately, there are various organic and chemical treatment methods to manage leaf galls and prevent major damage.

Identifying Leaf Galls

The first step in dealing with leaf galls is learning how to identify them. Here are some tips:

Examine leaves closely for abnormal bumps, swellings, or growths. These may appear light green, white, yellow, red, or brown.
Look for galls on the undersides of leaves too. Some species create galls only on the lower leaf surface.
Note the gall’s size and shape. Is it round, elongated, smooth, hairy? The appearance provides clues to the causal organism.
Check if the gall is solid or hollow inside. Slice it open carefully with a sharp knife.
See if there is an opening or hole in the gall, from which insects may have emerged.
Compare to pictures online to identify the pest. Common types are gall midges, gall wasps, gall aphids, etc.

Knowing the source organism is key to effective treatment. Consult an extension agent if you are unsure of the identity.

Understanding Why Leaf Galls Form

Galls arise from the parasitizing organism injecting chemicals into the plant which disrupts normal cell growth. This causes the plant to quickly produce tumor-like growths around the parasite. The lump shelters and feeds the developing pest.

Galls form due to the complex chemical interplay between plant and parasite. Specific insects and mites inject different stimulants, causing varying gall shapes and sizes. Other organisms like fungi and bacteria directly attack plant tissues, triggering uncontrolled swelling and growth.

So galls are not infections, but actually the plant’s protean response to damage. Though unsightly, leaf galls are the manifestation of the plant trying to contain and survive the parasitism.

Evaluating the Damage Caused

Assess how much damage the leaf galls are causing before undertaking treatment:

Count how many leaves are affected and to what extent. A few scattered galls cause minor impact. Widespread infestation stresses the plant more.
Check if galls are localized to just a few branches or spread across the canopy. Damage is worse if widespread.
See if plant vigor or development seems affected. Stunted growth indicates a heavy infestation.
Look for early leaf drop due to galled tissue dying off prematurely. This also stresses the plant.
Consider the plant’s overall health. Impact will be worse on weaker or stressed plants. Healthy vigorous plants can better tolerate damage.
Identify the causal organism, as some are more serious than others. For example, gall wasps are very damaging to fruit crops.

Organic Methods to Control Leaf Galls

If the infestation is moderate, try these organic remediation methods first:

Manual Removal

This works best for large solitary galls:

Simply prune away affected leaves and stems taking care to remove all the gall tissue.
Destroy the galls by crushing, burning or submerging fully in water to kill the larvae inside.
Monitor for regrowth and remove any new galls promptly.

Maintaining Tree Health

Keep trees vigorous to minimize impact of galls:

Water and mulch adequately especially in summer to reduce plant stress.
Fertilize lightly in spring with an organic all-purpose fertilizer to improve vigor.
Prune selectively to open up canopy and increase air circulation.
Monitor for other issues like diseases, pests, etc. and treat accordingly.

Biological Control

Beneficial insects can be introduced to naturally control some leaf galling pests:

Predatory mites and tiny parasitic wasps help reduce gall midges and gall wasps.
Release lady beetles to feed on gall aphids like the cottonwood petiole gall aphid.
Always identify the pest first and select predator/parasite that specifically targets it.
Time releases for early spring when pests are most vulnerable. Follow supplier instructions.
Multiple releases may be needed for adequate ongoing control throughout the season.

Horticultural Oils

Applying horticultural oils can smother gall-making pests:

Use lightweight summer oils as a dormant spray just before spring bud-break. Thoroughly coat branches.
Repeated applications may be needed through spring when insects become active and lay eggs.
Ensure complete coverage on undersides of leaves where pests hide. Spot treat visible galls.
Oils work by coating insects on contact and blocking their airways. But they have minimal residual activity.
Oils also help deter fungal spore germination. So they offer some control of fungal leaf galls.

Insecticidal Soaps

Soaps disrupt cell membranes of soft-bodied insects causing dehydration and death. Effective for aphid and mite galls:

Spray early as a preventive treatment before galls start appearing.
Obtain good coverage of leaf undersides where the pests congregate and feed.
Repeat applications every 5-7 days to target newly emerged adults before they reproduce.
Works best on younger stages. Older instars with protective waxy coverings are harder to control.
Avoid applying soaps during hot sunny weather or it may burn delicate plant tissues.

Neem Oil

Derived from the neem tree, this oil interferes with insects’ hormones and ability to feed/breed:

Use neem as both a preventive and curative to deter gall formation.
Spray leaves thoroughly, especially midribs, petioles, undersides where mites hide.
Multi-site mode of action makes it less prone to pests developing resistance.
Neem oil also suppresses fungal galls by inhibiting spore production and germination.
Can be combined with other organic oils or sprays for improved efficacy on galls.
Avoid applying to plants stressed by drought, heat etc. as it may cause leaf burn.

Chemical Control Measures

For severe infestations, consider using these chemical options:

Insect Growth Regulators

These IGRs disrupt insect development and prevent maturation/reproduction:

Azadirachtin – Derived from neem oil, it is non-toxic to mammals and beneficials.
Kinoprene – Mimics insect growth hormone, stunting larvae before they can damage plants.
Diflubenzuron – Interferes with chitin formation, inhibiting molting and growth.
Time applications carefully based on the gall maker’s life cycle. Multiple treatments are often necessary.

Systemic Insecticides

These travel within the plant & kill insects that ingest the treated foliage:

Imidacloprid – Taken up through roots and leaves, providing long-term protection against sucking/chewing pests. Avoid applying to blooming plants.
Thiamethoxam – Quickly moves to leaves & offers residual control of several weeks. Do not apply to drought-stressed plants.
Clothianidin – Absorbed rapidly and transported upward in the xylem. Works on many mites, aphids, beetles.
Follow label directions to calibrate dosage and timing. Resistance can develop with repeated overuse. Rotate between insecticide classes.

Miticides

To control mites that cause galls on leaves and buds:

Abamectin – Disrupts nerve impulses of mites leading to paralysis and death. Avoid direct sunlight after spraying as it degrades the chemical.
Fenpyroximate – Inhibits energy production in mites. Also effective on eggs and immature stages. Apply as mites appear, before populations build.
Bifenazate – Targets young mites during early stages of gall formation before damage occurs. Spray thoroughly for full coverage.
Rotate between miticides and don’t treat more than twice consecutively with the same product to prevent resistance. Combine with horticultural oils to boost efficacy.

Fungicides

Useful for combating fungal leaf galls caused by rusts, powdery mildew etc:

Copper-based products like copper sulfate – Work by producing reactive oxygen molecules that damage fungal proteins.
Chlorothalonil – Multi-site contact fungicide that prevents spore germination and growth.
Mancozeb – Broad spectrum fungicide that blocks spore production and has residual activity.
Begin applications at first sign of fungal galls. Reapply every 7-14 days as a protective coverage. Ensure thorough leaf spraying.
Alternate between fungicide classes to avoid pathogens developing resistance. Combine with leaf pruning to improve penetration and effectiveness.

Preventing Leaf Gall Recurrence

Alongside treating active infestations, take these steps to minimize future gall problems:

Maintain overall tree and shrub health through proper watering, fertilization, pruning. Healthy vigor helps withstand damage better.
Prune out and destroy gall-infested stems, buds and leaves to eliminate pests before they spread.
Clean up and remove fallen leaves/debris where pests may overwinter.
Use dormant oil sprays in late winter to early spring to suppress overwintering eggs and spores.
Apply sticky barrier bands around tree trunks to trap crawling insects before they reach foliage.
For severely susceptible trees, consider preventive application of systemic insecticide early in the season.
Rotate between pesticide classes rather than repeated use of the same chemical.
Introduce predator/parasite species to biologically control specific gall-making pests. Release multiple times for adequate control.
Monitor and quickly treat any early galls to prevent exponential population growth as the season progresses.

When to Seek Professional Help

If severe gall infestations persist despite your treatment efforts, seek professional arboricultural help. Arborists have specialized tools and expertise for issues like:

Difficult pest identification if the culprit is unclear. They can also test plant samples.
Selective spot applications of systemic chemicals where infestations are localized to certain branches. Avoiding whole-tree treatments reduces environmental impact.
Gall pruning and removal in tall trees which is challenging or risky for nonprofessionals. Their specialized equipment allows safe access to the high canopy.
Multiple complex pest problems attacking the tree simultaneously, for example gall wasps and fungal leaf spot in tandem. Professionals can assess complex situations and tailor multi-pronged treatment plans.
Large mature trees where thorough gall treatment requires commercial-scale equipment like hydraulic sprayers. Homeowner-grade products may be inadequate for full coverage.

Seeking timely professional help can get the infestation under control before lasting damage occurs. For long-term management of gall-susceptible trees, committing to annual expert care may be warranted.

Frequently Asked Questions About Leaf Galls

What causes leaf galls on my plants?

Leaf galls have numerous potential causes including insects, mites, fungi, bacteria, viruses, and nematodes. These organisms stimulate abnormal plant cell growth resulting in gall formation by injecting chemicals, feeding on tissues, or causing infections.

Should I be worried about leaf galls?

Not necessarily. While unsightly, most leaf galls don’t severely harm the plant unless populations explode. Assess damage levels – scattered galls are usually just a minor nuisance. But heavy widespread galls can weaken plants by distorting growth and clogging vascular tissues.

How do I remove leaf galls?

For large solitary galls, prune away the affected leaves/stems. Remove every bit of the gall tissue and destroy it so pests can’t escape. For small or clustered galls, prune off entire galled branches or use spot pesticide applications targeting the infected areas.

Are leaf galls contagious to other plants?

Not directly. But insects and mites emerging from fallen leaf galls can spread and initiate new galls on other plants and trees. Fungal and bacterial galls can also spread via blowing or splashing spores. So sanitation is key – remove and destroy galled debris.

How do I prevent leaf galls from coming back?

Promote plant health to withstand minor damage without stress. Prune out galls promptly before pests spread. Apply dormant winter sprays of horticultural oils. Preventively treat susceptible plants with systemic insecticides early in the season. Release biological control agents like predatory mites.

When should I call an arborist about leaf galls?

Seeking professional help is advisable if you are unable to identify the pest, the infestation is widespread, higher canopy areas are unreachable, the galls persist despite treatment, or other complex issues like diseases/pests are also involved.

Conclusion

Leaf galls can be managed through vigilant monitoring, properly identifying the causal organism, and utilizing a combination of organic treatments, chemical sprays, and prevention techniques. Remove and destroy galled plant tissues promptly to prevent spread. Prioritize cultural practices that maintain plant health and vigor, as stronger plants better withstand pest pressures. Seek expert arboricultural help for severe or complex gall infestations. With persistence and repeated thorough treatment of new and old galls, their recurrence can be minimized. A proactive integrated pest management approach is key to successfully protecting plants from serious leaf gall damage.

How to Deal With Leaf Galls