There have been so many calls recently with the same problem,  thinning canopy with dieback.  Thiscan be caused by several problems, but the most common this time of year is  lack of good water management over the  year, accentuated by lack of rain and salt damage.  The rain will often leach salts accumulated from the root zone over these many years of water from only irrigation.  I thought as everyone else did that we would be having some gushers in S. California and some of these problems would be relieved.  With rain, the cankers from black streak, bacterial canker and Botryosphaeria canker (Dothiorella) staining are washed away, and often  times, so are the tree's problems.  Well that hasn't happened. Or rather the rain hasn't happened to resolve this problem - Botryosphaeria or Bot Rot.

This problem has been ongoing and growers are still calling in about avocados with thinning canopies, fruit drop and sunburn and leaf death. Coastal avocados are always difficult to irrigate.  Mild weather followed by dry windy conditions means growers have to scramble to get water on.  If the trees are on some sort of calendar schedule, it usually means the trees get stressed.  If trees are on a slope where pressure is not properly regulated, some of the trees are going to get stressed.  If emitter clogging is not addressed, then more trees get stressed.  And lack of rains to leach salts from the root zone, and more trees are stressed. 

This stress sets up the trees for disease and a very common one in an avocado orchard that is filled with lots of leaves where decay fungi are working is stem and leaf blight.  The disease causes defoliation and exposed fruit sun burn and drop.  In an orchard, it's possible to see healthy trees and sick ones at the same time.  This may be due to the differences in soil type from tree to tree or the fruit load on trees - more fruit, more stress.  Looking out over the orchard there may be a polka dot of sick trees.  And it might all happen at once in a week or gradually.

So, this is a problem that is out there, if the irrigation issue is corrected, the trees usually recover.  It might require white washing and pruning out dead tissue.  If it is a young tree under two years, it might actually kill the little tree, but the disease is not usually fatal, just loosing the fruit.

This is a disease that goes to many different tree species - redwoods, eucalyptus, pine, Brazilian pepper, CITRUS.  The cause is the same, water or salinity stress.  To read more about this disease, go to:

http://www.ipm.ucdavis.edu/PMG/r8101311.html

Thinning canopy, dieback and shriveled fruit in avocado and dieback and gumming in citrus