Plant Diseases/Help with Avocado Tree
I was wondering if this tree is save able, if so, what ratio fertilizer do you advice? The tree is about forty years old and does not bear good fruit, maybe one fruit out of fifty. Any help would be wonderful. Thank you very much!"
It is hard to know the exact problem with the tree based on your images. This could be a disease problem, but it may be nutritional. I can see at least one limb in your image that appears to be dying (yellow/brown). If you notice dying limbs, I would suggest that they are removed. Often, if you cut out the dying limb you can prevent diseases from reaching the main trunk which could kill the tree.
I cannot tell exactly what problem the fruit has on it from the image. This website does not support very large images. I would suggest that you take a look at this website and there are nice images of various avocado problems as well as tips on management.
It is possible that the problem could be related to fertility. Rather than trying to guess at what fertilizer to use, it would be much better if you had a soil fertility test done. I do not know what county in California you are in, but if you contact your local county extension office about a soil test, they can help you coordinate it. You basically sample from around the base of the tree, usually at least 6 inches down and mix several shovelfuls in a bucket. Then, about a pint of soil is sent in for testing. The cost of the soil test for nitrogen, phosphorus and potassium is usually $10-20 per sample. If you think about the replacement cost of a tree and the money you would spend on wasted fertilizer, it is really not that much.
This is a link where you can find your county extension office.
The other nice thing about calling them is that the Extension Educators can often help you figure out what problem your tree has. They are going to be most familiar with problems in your local area, so you should describe the problem to them and possibly take them a sample from the tree to be examined.
Good luck with your tree,