Bill Pramuk

Bill,

We have three ornamental pears at the front of the house. They flowered late January then leafed out per normal through February. I was out of town for a week recently and when I returned I noticed that the leaves were getting spotty and many had fallen. Over the weekend one tree nearly entirely defoliated, the one next to it is close behind and the other one looks like it’s creeping into the third. The leaves are spotty or dark brown.

At the same time there appears to be budding and new leaf growth around the bare buds.

Our landscape pest control company sprayed twice for fungus in the past month. I’ve made a call to them but haven’t heard back yet.

Do you have any idea what’s happening? I’m worried about these trees!

M. C.

The photographs with that email showed the three trees: evergreen pear (Pyrus kawakamii) with almost leafless branches and a scattering of fallen leaves with brown blotches and spots.

Once a common choice for Bay Area landscapes needing a broadleaf, evergreen, flowering tree, evergreen pear has slipped in popularity. I think it is largely a result of its susceptibility to this fungal leaf spot disease: Entomosporium leaf spot.

Examples of evergreen pear are growing on the Soscol Avenue median between Lincoln Avenue and Vallejo Street. They, too, show symptoms of the leaf spot disease.

I first learned of this disease in the late 1970s when Indian hawthorn shrubs, popular because of their drought tolerance, began to show leaf spots and defoliation, especially in certain, susceptible varieties and where the plants received overhead irrigation.

What I am getting at here is the “Disease Triangle”:

1. The disease organism. Many are practically ubiquitous in the environment.

2. Conditions favoring infection.

3. A susceptible tree or plant.

If one side is absent, there is no triangle; no disease.

In this case the organism is a fungus Entomosporium maculatum, which is known by various scientific names. Depending on the reference it may also be named Dilpocarpon or Fabraea. The disease infects numerous plant species and is found world wide, so eradication or exclusion are not practical options for control.

Concentration of the fungus builds up on accumulated infected, dead, and dying leaves, on the twigs of infected trees and where susceptible species are in close proximity.

Conditions favoring infection include:

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— Water splashing and dripping on leaves, which could be irrigation spray or rainfall.

— Moderately warm temperatures. The fungus grows most rapidly at 68 degrees Fahrenheit and the spores germinate best at about 71-79 degrees.

Hot temperatures, 86 dgrees and higher inhibit its growth. So, in principle, mild, wet conditions favor the disease, while hot and dry conditions inhibit it. How this plays out in any given microclimate is hard to tell.

Genetics strongly influence inherent susceptibility of particular tree species and cultivated varieties. I would be interested to find any references documenting resistance induced by good nutrition, but have not seen any such thing for this disease.

Good sanitation — cleaning up and removing fallen, infected leaves — makes good sense to reduce the concentration of the fungus near the tree, but it does not assure reduced infection.

Appropriate irrigation — no aerial spray irrigation wetting the leaves — also makes good sense, but that is not the typical route of infection in evergreen pear trees.

Fortunately, Entomosporium is not a tree killer. Infected leaves are shed in spring and early summer. New leaves emerging in warm dry weather typically remain clean, at least through the remaining portion of summer.

I have seen evergreen pears kept relatively free of leaf spot but only with several timely applications of protective fungicide sprays.

Most folks I know, dealing with this problem, choose benign neglect: Let nature take its course, clean up fallen leaves, and wait for warm, dry summer days.

Bill Pramuk is a registered consulting arborist. Visit his website, BillPramuk.com, email questions to info@billpramuk.com, or call him at 707-226-2884.

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