Get ready for more rotting avocado fruit if you have leaf blight showing up in your tree canopy. The fungal spores (one of the Botryosphaerias we once lumped as Dothiorella) that create the infection spread in an irregular pattern over the leaf and down the stem (then called “stem blight”). This is often confused with salt or tip burn. The two conditions are caused by the same problem, water and or salt stress. However, in the case of leaf blight, this is a pathogen that can pass to neighboring fruit and begin the process of rot. This starts happening when the fruit starts ripening and softening, so it's often not seen in the orchard, but the packhouse or in the market.
Control is basically gaining control over the soil moisture and salinity in the root zone and when the leaf blight starts showing up in the canopy, cutting as much out back to green tissue as is economically possible.
Leaf blights from this group of fungi have also been reported as infecting other fruits, such as citrus, apple, peach and grape among others. The solution is the same - water right and cut the stuff out when and if it shows up.
Rot spreading to flesh
This has been a hot time. Look for irrigation problems, but also look for other weather related problems. This is possibly the most shocking.
If you see your citrus tree suddenly collapse. It happens on a weekend or in a few days, and it has suddenly turned hot with a Santa Ana or Sundowner, it is quite likely "Dry Root Rot". This is a situation where the water conducting tissues, the xylem, have been clogged by fungi and when a sudden demand for water comes from the weather, the tree collapses because it can not support the movement of enough water to satisfy that demand. It is more common along the coast than the Valleyk, but it does occur there, as well.
It starts with some type of wound to the crown roots or stem - gophers or field mice nibbling away at the roots or crown. Or damage from weed whipping or some sort of mechanical damage. This allows entry of the Fusarium fungus that causes the xylem clogging. You can usually see the damaged tissue just below the bark, near the base of the tree. The damage has usually occurred several years prior and takes awhile for the fungus to do its clogging. There's no know cure for this situation. Cut the tree out. It's not particularly contagious as far as we know. The fungus is cosmopolitan, meaning it is found in most orchards, and it just waiting for a wound to invade. You can replant into the orchard, but just avoid causing wounding.
Dry root rot affects all citrus species and varieties.
Collapsed canopy, grapefruit
Dead tissue below the bark
By way of Ventura grower Sandy Hedrick. This is a compilation of nights of "firing" done by Farm Advisor Bob Brendler from 1923 to 1988 that was collected by Terry Schaeffer of The Fruit Frost Service. Can you imagine over 70 nights of freezing temperatures when the smudge pots had to be lit? Now we don't normally "fire" the pots, but use water and wind more commonly. And it's only for a few nights at the most during the winter. In the "old" you couldn't plant avocado or lemon east of Sycamore Rd, just east of Santa Paula. Now there are avocados planted in Fillmore. And apple bloom erratically out there for lack of chill hours. And in summer it's getting too hot to grow strawberries even on the coast because with temperatures above 80 deg there is poor pollination.
Brendler loved weather information and irrigation issues. He was the first person in the Ventura office to use a computer and he was the oldest Advisor in the office at the time. The weather data chart was the first type of chart that could be done on a computer.
Do you think something has changed? By the way Terry is still running a private weather service for growers in Ventura.
Click on the link: Temp Chart002
We are creatures of habit and when we see the effects of a treatment, we can often persist in seeing the same or similar symptoms and assuming the cause is the same. In a recent case, a newly planted ‘Pixie' orchard, planted in August had gone into an old ‘Valencia' ground. The trees went through an adjustment period, but still didn't look sprightly in the fall. The grower applied a hand application of urea on the rootball that within 10 days had caused the trees to go into a salt swoon. Meaning, they got too much fertilizer that burned them. The grower seeing the effect, immediately started the sprinklers, but the damage was done. Several months later the trees had either died or were still lingering, but hanging in there. The trees were still coming out of winter, but the trees hadn't perked up. It was a dry winter and some of the yellowing was due to underirrigation, that overall yellowing from lack of nitrogen and the leaves were curled. But the assumption was still that the trees were recovering from the salt burn from the urea.
Looking more closely at the trees, something else was odd about some of the trees that were continuing to die. The leaves suddenly wilted. Getting down on hands and knees and digging around the roots, there were few roots and ………………………….a tunnel. A gopher had been at this tree and the lack of roots were probably due to Phytophthora root rot. Looking around there were some old ‘Valencias' that had been hit by gophers and there were gophers mounds and runs all over the place.
So, young trees planted in the heat of the summer into root rot ground with gophers waiting in anticipation that had been salt burned in a year with little rainfall. A lot of causes for trees that generally weren't happy – triste, as they say in French.
So what are the lessons here? Avoid old citrus ground when planting with citrus, and if you can't make sure, don't plant in a stressful period. Phytophthora loves stressed trees and adding lack of rainfall an gophers and salt, just heightens the stress. Make sure to get the irrigation right. Don't irrigate them to the schedule of the older trees and start them off on one of the phosphite materials.
Wilted, yellow leaves from lack of water and Phytophthora
Gopher chewing on stem
Gopher run and lack of roots from Phytopthora and gopher
Gopher mounds in planting area
Older Valencias dead and dying from Phytophthora and gophers
Water woes are probably not going to go away, so readup on how to best manage water at this new blog.
- Author: Rachael Long
Guest post from Rachael Long, UC Cooperative Extension Farm Advisor, Yolo County
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