When you look at your pear tree leaves, black spots are never a good sight to spot. It’s alarming, because it’s almost always a sign of a bacterial or fungal infection.
Identifying the cause isn’t always straightforward. It’s easier to know what’s going on when the black spots are on the fruits. If it’s a black smudge that wipes away, that’s likely sooty blotch. Accompanying that is flyspeck, which is a cluster of tiny black dots on fruit.
When the black spots are on pear tree leaves, that can be mystifying!
Fungal and Bacterial Diseases that Cause Black Spots on the Leaves of Pear Trees
Fabraea Leaf Spot
This is the most common culprit.
Fabraea leaf spot is a fungal infection that affects a range of fruiting trees from early spring to late summer. This is not the same as bacterial leaf spot. The damage it causes is easy to identify.
The leaf spots start out as pinprick sized black dots that soon expand to black spots about 10 mm in size. You won’t have one black spot. You’ll see clusters of them.
A simple way to tell it’s this fungus is that they’re all uniform in size. Up to 10 mm on the leaves, bigger on the fruits, and they will appear on your fruits, too. Just bigger.
Depending on when you spot the spots on leaves, they can appear larger. The fungal spores are distributed by splashes of water causing clusters to form close together.
As such, the spots can run into others, creating what appears to be bigger black spots. Bigger spots are likely just multiple small spots joined together.
The middle of May to late July is when fruit trees are most susceptible. The hotter the temperature, the faster the spread. When temperatures are above 50oF, wet foliage risks this fungus attacking your plant.
Fabraea can overwinter on fallen debris so you can get secondary infections.
To treat fabraea leaf spot, the leaves need to be pruned from the plant, and fallen debris kept cleared from the ground below your tree to prevent the wind from blowing the fungal spores back on it.
Pear Psylla gives you a two-pronged problem. These start as nymphs that morph into flies. The pear sawfly.
The first is in their larva stages when they’re a teeny pear slug. The second is when that slug matures into an adult sawfly, that’s like a black wasp, but it won’t sting.
These insects inject a toxin in the leaves. The injection sites darken first because of the toxins and the rest of the leaves turn yellow.
If you’re seeing darkened black spots and yellowing leaves on your pear trees, there’s a chance there’s a Pear Psylla slug feeding on it.
The adult sawflies do not feed on the plant tissue. They’ll instead use it as a nest by depositing their eggs. The leaves of pear trees are a source of nutrition for nymphs.
You’re more likely to find the tiny black slugs feeding on the underside of leaves. Their appearance is different from most slugs as they have a wider top tapering at the bottom.
They’re also shiny black. Not the typical horrible shade of gunky brown most other garden slugs have.
Knock on the leaves and these fall off, or pick them off by hand. Treat the leaves for emerging nymphs with a regular application of horticultural oil.
Light Frost Bite
Light frost bite is only a cosmetic problem to start with. However, it still warrants your attention because when the leaves get damaged by a light frost, the pores on the leaves get frozen open, leaving them at higher risk of bacterial and spores infecting them.
A sign of frost bite is discoloration that’s contained to the tips of leaves. The less severe the damage, the more it’ll look like a tiny black spot that’s contained to the tip of pear leaves with no other part of the leaves damaged.
A light touch of frost, such as a cold wind brushing the canopy, could be mistaken for black spots on the tips of pear tree leaves.
Light frost damage in spring typically causes this weird burning look. It’s not the same type of damage that fire blight causes, which is full leaves blackening.
Instead, only the tips show signs of burning. The lighter the frost hits it, the smaller the black tips are.
The good news is there’s no treatment needed. The bad news is that it’s a clear warning to keep an eye on the next few weeks for fungal spores that infect the weakened tips of leaves.
Blossom blast is likely to infect pear trees already damaged by frost. It’s caused by Pseudomonas syringae, a common bacterial pathogen that’s spread by cold and wet weather.
The most damage this does is to the bottom part of pears and surrounding leaves. The calyx turns black, and it kills off clusters of leaves near the tip. However, nearby younger leaves can develop deep black spots.
The bacteria spores are spread by the weather and by insects. It needs open spores to infect leaves and since early spring frost damages the leaves, it’s in early spring that you’re likely to find this to be a problem.
If you do come across sporadic blackening that’s akin to fire blight, but contained only around the fruit blossoms, it’s likely this bacterial infection.
This tends to be more prominent near the calyx of pear fruits, but on young tender leaves, this can show as small black depressed spots on tender leaves of pear trees.
Pear scab is another fungal infection caused by the pathogen Venturia pirina. This causes black lesions on pear tree leaves, but that’s only the start of what can become a much bigger problem.
Pear scab starts with the youngest, most tender leaves on pear trees. Once it gets a foothold, it’ll put a stranglehold on all parts of the pear tree. The leaves, blossoms, fruits and shoots. No part of the pear tree is immune.
Pear scab lesions on leaves are black spots that are under 10 mm in diameter. Unlike the black spots on pear tree leaves caused by fabraea leaf spot, pear scab lesions are not uniform in size, nor are they perfectly rounded. This causes black spots with ragged edges.
Like fabraea leaf spot though, this fungus is spread by rain and it can overwinter in the ground, too. The most susceptible parts of plants are the youngest leaves. That gives this fungus a starting ground.
Any infected leaves should be pruned, discarded and the leaves should be treated with a suitable fungicide.
How to Treat Black Leaf Spots on Pear Trees
Prune with Intention
Gardeners prune their pear trees to encourage more growth, which is terrific for growing more fruit. Too much growth concentrated in one area though is a breeding ground for fungi and bacteria.
When you’re pruning any fruiting tree, don’t just think of it as encouraging growth. Look to see what’s on the ground.
Is there anything there that could be harboring pathogens that the rain can splash onto the leaves? How about nearby plants that could cause fungal pathogens to blow over and onto your fruit tree?
When pear tree leaves get black spots, it’s almost always a bacteria or fungal disease. Wet and warm conditions help them spread. A lack of air circulation helps them thrive. For that reason, prune your fruit trees to thin them out.
Good air circulation is critical for the leaves to dry faster after a downpour. Wet leaves in warm weather never ends well.
Overhead Watering Is a No-Go
Wet and warm climates are what breed fungal and bacterial infections. For that reason, keep your leaves as dry as possible.
The simplest way to do that is by using a drip irrigation system. If you aren’t using that technique, then pay attention to the amount of rainfall.
Pear trees only need an inch of water per week, and should be watered close to the trunk. Not on the leaves. Water at ground level to nourish the roots and let them do the work of distributing the water.
Copper based fungicides are effective at bringing fungal infections under control and preventing their re-emergence. You have to be careful with the dosing though.
Every product is different and will have varying dilution requirements. While they can all penetrate deeply into plant tissue to treat fungal diseases; too strong of a solution can burn the plant leaves.
When applying copper-based fungicides, apply it directly to the leaves that need protecting. Don’t put it in the soil because it won’t break down like fertilizer.
Copper-based fungicides are only beneficial at preventing black spots from reoccurring. They won’t fix the calluses already on the leaves.
When you see a pear tree leaf with black spots caused by a fungal infection, cut the whole leaf off and discard it. Use copper fungicides to stop black spots from appearing again.
Many funguses can survive on the ground, causing a secondary infection when either wind blows spores onto lower and new leaves as well as splashes of rainwater springing spores up from the ground onto your plants.
That’s the purpose of fungicides. To stop a repeat of black spots. Not to get rid of the ones that are already on the leaves.