By: Mary Ellen Ellis
Shot hole disease can attack several types of fruit trees, but apricot is especially vulnerable. This fungal infection, previously called Coryneum blight, favors wet conditions, especially in the spring, and causes damage to buds, leaves, shoots, and fruit. Preventative steps are the best measures for managing this disease.
Apricot shot hole fungus is Wilsonomyces carpophilus. It overwinters on buds that are infected and also on twigs. The spores on these parts of the tree can be transferred during winter and spring rains and when water splashes up from the ground. Those spores need 24 hours of moisture to set in and cause infection, so wet and humid conditions tend to lead to the spread of this disease.
Apricots with shot hole disease may show a variety of symptoms, but the name comes from spots that develop on the leaves and then fall out, leaving round little holes behind. The first signs of apricot shot hole fungus disease on trees in the spring are purple spots on the new shoots, buds, and leaves. The spots on leaves that become holes begin small and often have a yellow or light green margin.
Severe infections will cause the leaves to drop early, sometimes as early as in the spring. Extensive infection also begins to affect the fruit as it develops, causing scabby, rough spots that are concentrated on the top of the fruit and that may flake off and leave rough patches behind.
Treating apricot shot hole disease once it has become advanced is difficult. The best measures begin with prevention. The disease is most common in wet conditions, so making sure trees are well spaced for airflow is crucial, as is regular apricot pruning to allow for circulation between branches. Avoid irrigation that causes water to splash up onto branches.
If you do see signs of the disease, the best way to treat it is to apply an appropriate fungicide during the dormant season. This can help to minimize or prevent the disease from infecting healthy plant material in the spring and during the rainy and wet season. This can be done just after the leaves fall or right before buds break in the spring. You should also prune off and destroy and heavily diseased branches or twigs.
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Shothole is widespread in Maryland, all of my trees have a bit of it, so I don't think removing the tree is going to be of any help long-term. Just-planted trees are more susceptible to shot-hole and rains also make things worse. There isn't a lot you can do about shot-hole during the growing season, the main treatment is to spray copper at leaf fall and again just before the leaves come out in the spring. There is a spray called fungonil which can be of some good now if you can find it (its hard to find). It will moderate when the really hot weather comes. I would just leave it until the fall and hit it with copper then.
Thank you so much for the information. I'll look at some specialty stores for fungonil. It's good to know that it is something that may be treatable to some extent. I appreciate your time in responding to my post.
Mellisa, I had horrible shot-hole when I was planting out my orchard and I thought I would never get rid of it. But once I started to do the fall/spring copper I got it under control. With high-density plantings it is very important to keep them opened up since dense trees are much more likely to get diseases. The high-density technique developed in California does not directly work in the east. What it means is you need to keep very few scaffolds on your trees. I have many trees that are Y-shaped (two scaffolds) or just one narrow spindle.
You don't say just how close is "close" in your high-density planting, but you may be in for some surprises. It would help to know where you sourced these trees, and whether you know exactly what rootstocks are beneath them.
Some apple rootstocks, such as M-27, M-9, and Bud-9, can properly be defined as "dwarf", but if the apple rootstock is M-7 or any of the other semi-dwarfs, it is a different story entirely. Many mailorder and online nurseries will throw all of these into the same basket, assuming the grower is not interested or concerned with the differences. Often, you cannot trust nurseries to properly define a "dwarf".
Unless peaches are true, genetic dwarfs, they are usually on rootstocks like Lovell, Bailey, or Halford, and these are capable of growing trees well over 12 feet tall with equivalent spread. Once they reach the takeoff point at about 5-6 years, keeping them under control is a big, big job. Peach trees produce best when they are pruned to open centers and allowed to spread out. One good spreading peach tree can produce more peaches than 3 trees that must be constantly pruned to keep them from growing into one another. And the single tree will have fewer disease problems since it is open to light and air.
The same is true of apricot trees, which are normally budded on seedling roostocks. I have apricot trees in my orchard that would have easily reached over 15 feet if I did not regularly prune them down. There are not, as far as I know, any apricot rootstocks that will actually control the growth of these trees in a manner that could be called "dwarfing". The slowest-growing apricot tree I have now is a Puget Gold on a plum rootstock called St. Julian "A", and that is now over ten feet tall with an 8-foot spread, and wants to grow larger.
I know there are many advocates of the "pack'em in" school of fruitgrowing here, but I am not one of them. I would prefer to have one good, stand-alone tree that I can get around to spray, thin and pick easily, and don't have to be concerned about trees growing into a tangled mess. I think they look better too, if aesthetics is a concern.
It is surprising that an apricot tree planted only this spring would develop shothole fungus, and makes me wonder if the tree was not infected when your received it. I would not, BTW, plant Moorpark or Early Golden apricots in Maryland. I had these varieties for over ten years here, and they produced zero. Now I have Tomcot trees, and pick a bountiful harvest of apricots nearly every season. My climate is very similar to yours.
Shot-hole disease is a combination bacterial infection (Xanthomonas prunii) and fungal disease (Blumeriella gaapi and/or Cercospora sp.).
Shot-hole disease (on a laurel in the photos) is a combination bacterial infection (Xanthomonas prunii) and fungal diseases (Blumeriella gaapi and/or Cercospora sp.)
Circular holes in the leaves that eventually join and make larger holes. The appearance of shooting a shotgun at the shrub and causing multiple holes.
Laurels (bay and Otto Luyken), camellia, ligustrum (privets), hydrangea, ivy.
April through October, peak in May and September.
Leaves appear to be ‘eaten’ away by the disease, leaving a ragged appearance. As leaves are damaged, they begin to fall away, the plant looses its ability to make food and can become stressed.
Sanitation is the best way to keep the disease from coming back. Clean up contaminated leaves from under the plant. When diseased leaves build-up under the plant, rain or watering can splash the disease back up on the plant. Spray the leaves with Mancozeb, Kocide, Kop-R-Spray or other recommended products containing copper at the first sign of a problem. Always READ THE LABEL and DIRECTIONS FOR USE section carefully when using pesticides.