Red buckeye is a tough small native tree, and most of its common troubles show up on the leaves rather than in the trunk or roots. A tree that looks shabby by late summer is usually not a dying one. Part of what alarms owners isn’t disease at all but the tree’s normal habit of finishing its season early. The problem worth taking seriously is a different one: the seeds are poisonous if chewed and swallowed.
Brown Blotches on the Leaves Are Usually Leaf Blotch Fungus
The most common leaf disease of buckeyes and horse chestnuts is Guignardia leaf blotch, a fungus. It starts as water-soaked spots that turn into large reddish-brown blotches, often with a yellow edge. The blotches are often shaped by the leaf veins, and tiny black dots may appear in the dead tissue. Badly blotched leaves curl, dry out and may drop early in late summer.
Blotch tends to sit in the middle of the leaf as separate patches, while scorch browns the edges and tips first and works inward. Blotch is worse after wet weather and scorch after hot, dry spells. If you’re unsure, a hand lens may show the black dots in blotch patches, which scorch lacks.
The fungus spends winter in fallen infected leaves and releases spores onto new leaves in spring, so the first infections happen as buds open and new leaves unfold.1Center for Agriculture, Food, and the Environment at UMass Amherst. Guignardia Leaf Blotch: Landscape: Center for Agriculture, Food, and the Environment (CAFE) at UMass Amherst Spots on infected leaves then make more spores that spread the disease through summer.2Center for Agriculture, Food, and the Environment at UMass Amherst. Guignardia Leaf Blotch: Landscape: Center for Agriculture, Food, and the Environment (CAFE) at UMass Amherst Repeated rain with cool weather favors the disease,3Purdue Landscape Report. Guignardia Leaf Blotch so a cool, wet spring often brings a bad blotch year, and a dry one a mild year.
Blotched leaf tissue makes less food for the tree, since less of the leaf is working.4Arboriculture & Urban Forestry. Water-retaining Polymer and Fungicide Combinations Reduce Disease Severity Caused by Horsechestnut Leaf Blotch [Guignardia aesculi (Peck) VB Stewart] But most damage comes late in the growing season, after the tree has done much of its yearly growth. An established tree usually leafs out normally the next spring, and the same tree often looks healthy again.
How to Handle Leaf Blotch
Cleanup is the main control. Rake and remove fallen leaves from all nearby buckeyes and horse chestnuts after fall leaf drop.5Purdue Plant Doctor. Identify and Manage Guignardia Blotch on trees and shrubs Bag or bury them rather than leaving them under the tree.
Wait until most leaves are down, then rake in one or two passes. Include leaves blown under shrubs or into beds nearby. Don’t use infected leaves as mulch under the tree. Composting works only if the pile gets hot; otherwise bag them.
Fungicides such as mancozeb and copper hydroxide can improve the look of specimen trees and young nursery trees.6Center for Agriculture, Food, and the Environment at UMass Amherst. Guignardia Leaf Blotch: Landscape: Center for Agriculture, Food, and the Environment (CAFE) at UMass Amherst For many mature trees spraying is not practical, and it is not needed in most cases.7Center for Agriculture, Food, and the Environment at UMass Amherst. Guignardia Leaf Blotch: Landscape: Center for Agriculture, Food, and the Environment (CAFE) at UMass Amherst8Purdue Landscape Report. Guignardia Leaf Blotch Sprays work only as protection, so start as buds open and before bloom.9Center for Agriculture, Food, and the Environment at UMass Amherst. Guignardia Leaf Blotch: Landscape: Center for Agriculture, Food, and the Environment (CAFE) at UMass Amherst10Purdue Landscape Report. Guignardia Leaf Blotch One or two more sprays may be needed after bloom if rainy, cool weather continues.11Purdue Landscape Report. Guignardia Leaf Blotch
Read and follow the product label, which lists which trees and diseases it covers. Spray on a calm, dry day so the product dries on the leaves. Spraying after blotches appear will not cure spotted leaves. A small tree you can reach is a better candidate than a tall one.
Scorched Leaves and Early Leaf Drop
Red buckeye prefers partial shade and tolerates full sun,12NC State Extension Plants Database. Aesculus pavia but in full sun it may get leaf scorch in summer heat.13NC State Extension Plants Database. Aesculus pavia On hot, windy days leaves lose water faster than roots can replace it. Scorch is therefore worse in dry spells and on trees near pavement or walls that reflect heat.
For a scorched tree, water deeply once a week in dry spells rather than lightly every day. Soak the whole area under the branches, not just near the trunk. Young trees in their first few years need this most. Spread 2 to 3 inches of mulch out to the branch tips, kept off the trunk.
Browned leaf edges will not turn green again, so judge progress by new leaves next spring. Don’t fertilize heavily in drought, which pushes growth the roots cannot support.
Red buckeye is one of the first trees to leaf out in spring. It also often drops its leaves early, sometimes by late summer, especially in dry years. Check the buds and twigs, which should still be firm with plump buds for next year.
Leaf drop is more than normal when twigs snap dry and show brown inside instead of green, or when no new leaves appear the following spring. Bark that is cracked, peeling or oozing near the base is another warning, as is dieback that returns year after year in the same branches.
White Powder on Leaves Is Powdery Mildew
Powdery mildew shows up as white growth on leaves and stems, and buckeye is a common host.14Ohio State University. Important Tree and Shrub Diseases The white growth is the fungus itself growing on the leaf surface.15Ohio State University. Important Tree and Shrub Diseases Tiny feeding structures reach into leaf cells and injure them.16Ohio State University. Important Tree and Shrub Diseases Even so, powdery mildew usually will not kill a plant, though it makes it look much worse.17Ohio State University. Important Tree and Shrub Diseases
You can tell it from other problems by touch: mildew can be rubbed partly off with a finger, while blotch and scorch cannot. It is white or gray on top of the leaf, not brown dead tissue, and it often appears late in the season when nights cool.
Mildew thrives in shade, still air and dense growth, especially later in summer. Thin crowded nearby plants and avoid wetting leaves at night.
On a mature tree, spraying is rarely worth the effort. If you do use a fungicide labeled for mildew, it must go on before the mildew spreads.
Sticky Leaves and Bumps on Twigs Mean Scale Insects
Soft scales look like round bumps about 1/4 to 1/2 inch long on twigs, sometimes with a waxy coating.18Davey Tree. Scale Insect Identification & Control They suck sap and drop it as a sticky substance called honeydew.19Davey Tree. Scale Insect Identification & Control Sticky leaves, cars or patios under the tree are an early clue. Black sooty mold often grows on the honeydew.
Signs of damage include small yellow-mottled leaves, thinning canopy, leaf drop and branch dieback.20Davey Tree. Scale Insect Identification & Control A heavy, untreated infestation can kill the whole plant.21Davey Tree. Scale Insect Identification & Control
On a small tree, scrape off a few scales by hand or prune heavily infested twigs. Horticultural oil in late winter smothers overwintering scales. Spray in late spring when young crawlers hatch and are unprotected. Ladybugs and tiny wasps often keep light infestations in check.
Call an arborist if scale covers many branches, if dieback spreads despite treatment, or if the tree is too large for you to spray yourself.
Site and Care That Prevent Most Problems
Red buckeye does best in partial shade and moist, well-drained soil.22NC State Extension Plants Database. Aesculus pavia It also adapts to different soil pH and many soil types.23NC State Extension Plants Database. Aesculus pavia For a new tree, light shade from taller trees is ideal.
You can’t move shade for an existing tree, but you can add mulch and steady water. Keep lawn grass and string trimmers away from the trunk, and avoid piling soil over the roots. Water through the first two or three summers after planting.
Prune dead or crossing branches in late winter. Thin crowded growth to let air dry the leaves faster. Avoid heavy pruning, which stresses the tree.
When choosing a site, east-facing spots get gentle morning sun and afternoon shade. If the seeds worry you, plant away from play areas.
Are Red Buckeye Seeds Poisonous
All buckeyes contain bitter compounds, and the seeds are the most poisonous part. Eating them can cause vomiting, diarrhea, weakness and wobbly walking in people and pets. Leaves and young shoots are poisonous to a lesser degree.
Red buckeye was not tested for toxicity in animal studies, and other species differ.24American Journal of Veterinary Research. Toxicity of seeds of three Aesculus spp to chicks and hamsters25American Journal of Veterinary Research. Toxicity of seeds of three Aesculus spp to chicks and hamsters So treat red buckeye seeds as poisonous rather than counting on any species being safe.26American Journal of Veterinary Research. Toxicity of seeds of three Aesculus spp to chicks and hamsters
If a child or pet eats one, remove any pieces from the mouth and call Poison Control (1-800-222-1222 in the US) or your vet right away. Note how many seeds and when, and keep a sample. Seek care quickly if vomiting, weakness or stumbling begins.
To reduce the risk, pick up fallen pods and seeds in late summer and fall. Teach children the shiny seeds are not nuts to eat, and keep dogs that chew things away from fallen seeds.
Growing a Replacement From Seed
Buckeye seeds need a cold, damp chill period that mimics winter.27Karsten Nursery. Grow a Buckeye Tree from Seed: Your Complete Step-by-Step Guide Without it the seed may not sprout.28Karsten Nursery. Grow a Buckeye Tree from Seed: Your Complete Step-by-Step Guide Seedlings are ready to transplant at 6 to 12 inches tall, usually by mid-spring.29Karsten Nursery. Grow a Buckeye Tree from Seed: Your Complete Step-by-Step Guide Harden them off first by setting them outdoors gradually over a week to avoid shock.30Karsten Nursery. Grow a Buckeye Tree from Seed: Your Complete Step-by-Step Guide
Use fresh seeds, since dried-out ones often fail. Place them in damp peat or sand in a bag in the refrigerator over winter, and check monthly that they stay moist but not moldy. Or plant them outdoors in fall and let winter do the work. Wear gloves and keep seeds away from children while handling.