This appears to be old Anthracnose damage, a common late-season phenomenon on Japanese maple and other tree species. Several species of fungi can cause anthracnose diseases, infecting leaves earlier in the season (usually when it's cooler and wetter, as in spring) and causing patches of dead tissue. Despite this eyesore, they tend not to negatively impact the tree's overall health, and usually are less noticeable in years with drier weather. They do not need preventative treatment measures, nor will fungicides used after symptoms appear be effective. Maple cultivars may vary in their resistance to anthracnose fungi, though we do not have a ranking of which are which, especially since weather plays a large role in disease severity from year to year.

If your method of watering the tree involves wetting the leaves, either switch to a method that keeps them dry or irrigate early enough in the day that the foliage is dry by nightfall. While you certainly can't do anything about rain, keeping leaves dry can help minimize the spread of fungal spores. Only irrigate when the soil several inches below the surface is becoming dry; the goal should  be periodic but infrequent soakings only on an as-needed basis.

Salt damage among plants usually refers to more than just sodium-based salts (the kind we're most familiar with, table salt). "Salts" are a number of mineral-based compounds that have similar impacts on root health when excessive - namely, causing drought stress through root cell damage, because the mineral levels impede the root's ability to absorb enough water. Fertilizer minerals and the resulting residues are also referred to as "salts," so applying too much fertilizer can result in salt buildup in the soil if irrigation or rainfall isn't leaching out the excess. (That's not to imply that added watering would be a viable solution to counteract that problem, though, since too much root moisture can rot roots.) Different plant species have different levels of salt tolerance; maples as a group tend to have below-average tolerance, but we do not have information specific to Japanese maple. If you're following the dosing instructions for the fertilizer spikes, you're probably not over-fertilizing, but keep in mind going forward that most in-ground landscape plants (aside from potted plants, annuals, and plants producing a harvest) do not need routine fertilizer to be healthy. While over-fertilization damage would not look like the patchy spotting in your photos, it would begin to mimic drought stress, which would be the drying and crisping of the leaf tips/points.

Miri

On 10/05/2021 10:51 AM Ask Extension <<personal data hidden>> wrote:

 

 

You're welcome.