When It Gets Hot, It Gets Confusing
So I got called out to see an orchard that had dieback. Over the phone it sounded like “Dothiorella Leaf Blight” (//ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=10354). This symptom shows up when there has been water stress of some sort, over a period of a few weeks to months.
It can be a branch or a whole tree. It usually is due to a watering schedule that doesn't meet the needs of the plant. It is exacerbated by competition from more aggressive plants nearby, like eucalyptus windbreaks. With increased competition over time, root rot can also start taking its toll because of the stress, as well. Where you see areas cleared of trees, and where there are struggling trees in various stages of survival, it's a good indication of a rough environment.
Trees with root rot symptoms may have been stumped previously, and with their weakened root systems are more prone to leaf blight.
In the case of this grove, it was at the top of the hill where it got all the wind and a full day's sun above the typical coastal fog.
So the grower asked about the damage. There was the heat damage from the July heat wave.
Then there was the damage that had showed up earlier, the leaf blight that was associated with the ongoing water stress. In a way, both of these symptoms are of water stress, one very rapid, hence the reddish tint to the leaves. The other a slower collapse, the leaf blight from prolonged water stress.
Over time, this type of stress had allowed root rot to take hold and trees were stumped in order to help them survive. The trees were also started on a regular phosphonate program. In general, the rest of the orchard looked good, except for this area near the competitive windbreak trees. The heat damage was mostly in this area, as well. The grower had been forewarned of the impending heat and irrigated the trees prior to the hard weather.
In the case of heat damage or blight, and leaf/stem blight, it's not fatal for mature trees. Younger trees under two years can be killed. They usually need whitewashing to prevent sun burn damage.
But even they can be revived if the watering is improved.