Q. Can foliar spraying micronutrients like iron on fruit trees cause bacterial leaf spot to form or spread? I have had problems with bacterial leaf spot on my nectarine trees but last year the leaves weren't affected as much. I don't want the pathogen to spread again.
A. Where do you live? Who made this diagnosis? I have not heard of bacterial leaf spot forming in the dry desert. In more humid climates it can be more common. The leaf spots on leaves should be surrounded by a yellow halo if it is bacterial leaf spot.
|Some iron sprays can cause staining on developing fruit and can resemble disease. This is iron staining quince fruit. Use iron chelate sprays if fruit is present.|
Bacterial infections usually require wet or humid environments and warm to hot temperatures or insects to spread the disease. And these wet/humid environments must be present over a long enough period of time usually many consecutive hours.
It would be possible if the plants are crowded together so that the leaves cannot dry or there is an air movement to help dry them. Overhead irrigation such as sprinklers could also contribute to a problem.
However, applying foliar micronutrients early in the morning so the leaves have a chance of drying before nightfall there should not be a problem. And spray early in the season.
|Some diseases like shot hole fungus are prevented with properly timed fungicide applications containing copper. Copper fungicides can also be used to help prevent some bacterial infections like bacterial Leaf spot.|
The pathogen or bacteria are already present on stems and move to the foliage early in the spring. The leaves just need the right environment (temperature and humidity or degree of wetness) to cause infection.
It also helps if the canopy is not dense so there is air movement to help dry the leaves. In your pruning you should create enough space between branches to allow for some sunlight to penetrate inside the canopy and reduce shading.
You probably have been told that there are not many pest control products for bacterial diseases but a fairly common fungicide like Bordeaux can give you some good protection if applied when temperatures get warm. Some of the disease potential can be removed through pruning in the winter.