- Author: Gerardo Aldunate V.
- Author: Mary Lu Arpaia
- Author: Ben Faber
Can't Say It Often Enough
Advice from Index Fresh
POST HEAT TREE MANAGEMENT
- Why are avocado trees so heat sensitive? Water is taken up by the roots and is essential for overall tree metabolism and carrying nutrients and metabolites to the growing portions of the tree canopy. It also serves an important function for overall temperature regulation for the tree since water vapor diffuses through the leaf stomata as part of the photosynthetic process. We know that avocado stomates close when temperatures exceed about 90F; this is modulated by relative humidity. When the stomates close, we break the process of water transport within the plant and water vapor is not released, thereby stopping the process of cooling.
- Think about it this way, the leaves act like a radiator to cool the whole tree. When stomates close, the radiator no longer functions, and the tree heats up.
- During high temperature events, especially with low relative humidity, the tree will have difficulty in taking up sufficient water to support the tree canopy, therefore it is important to hydrate the tree prior to predicted heat events to reduce heat stress.
- Overheating of the tree may lead to branch dieback, leaf scorch and fruit drop.
- Irrigation and Fertilization: Check the soil moisture level! The amount of water and fertilizer should be adjusted according to the level of damage of the canopy. With leaf drop, less water is transpired and the typical irrigation program needs to be adjusted. If you have to reduce the water, it is always better to reduce the amount of water applied and maintain the irrigation frequency. Always check soil moisture to verfiy irrigation needs!!!
- Pruning: Similar to a freeze event the immediate damage may not be visible for several weeks so resist pruning the tree. Once ten days to two weeks have passed any damaged branches or leaves will be visible which can then be removed. Remove only damaged wood from the tree. If you have any doubts, do not prune. You should keep new growth (especially summer shoots) as much as posible for the next year´s flowering. Dried leaves on the tree can provide sunburn protection, so resist pruning.
- Sun Protection: Whitewash immediately (same day) against sunburn.
- Notes: Work should be performed during “normal” temperatures. Pruning tools must be cleaned and disinfected between each tree especially in areas and/or blocks with a known presence of diseases, such as sun blotch.
Disease control: Prevent “wood diseases” like Avocado Branch canker. Remove damaged branches before the winter rains arrive in November. Remember, Pruning tools must be cleaned and disinfected between each tree
Just When You Thought You Could Take the Week Off from Webinars
Here's a two-day conference on Dragon Fruit
September 22 and 23, 2020 (you have an option of viewing it live or later recorded).
Note Taiwan is 15 hours ahead of California.
To register for this conference follow the link: FREE registration.
Please use the following link for the agenda
Besides the presentations on export there will be presentations on:
- Good agricultural practices
- Nutritional and functional traits of dragon fruit
- From production to consumption-the missing links
- Value-added programs
In addition: There is a lot of information on dragon fruit production through the
CRB Announces its
The Citrus Research Board (CRB) in coordination with the University of California Statewide Integrated Pest Management Program (UC IPM) is rolling out a new CRB Webinar Series, geared toward citrus growers and industry professionals.
Make sure you mark your calendars for the rest of the talks in the series.
For more information, please contact
Petr Kosina, with UC IPM, at firstname.lastname@example.org
Fruit drop happens soon after, or slowly, depressingly over several weeks after a heat wave. The stem turns brown, cutting off moisture to the fruit, and they drop.
If you see significant leaf drop in your groves due to excessive heat, the following actions are recommended:
- As soon as possible, whitewash branches exposed to the sun with special attention paid to branches on the west and south sides of the tree.
- Trees that lose a significant portion of leaves cannot efficiently move water, therefore restrict irrigation amounts to ensure you avoid creating wet, soggy conditions that can lead to root rot. It's best to irrigate less frequently and with smaller amounts of water.
- Do not prune your trees — leave hanging leaves in place to protect the tree from sunburn. Once new tree growth has occurred (in the next 3 – 6 months), pruning can take place on living wood.
- Adjust fertilization as you would with a frost-damaged tree: reducing the amount of fertilizer until the tree is re-established. If you see signs of a particular nutrient deficiency, adjust fertilization accordingly.
For more information about managing heat in avocado groves, growers can view the following articles on the California avocado growers' website:
So Sphaeropsis tumefaciens causes galls, but it's also been noted to cause witches broom in citrus, as well. Thjs type of broom formation is a wild, though compact growth. It's a dense mass of shoot growth from a single point – a messy mass that even a witch would have a problem using to sweep up. This is a symptom of infection by certain fungi, like rust or mildew or a genetic mutation in the plant itself. If it's a stable genetic change that doesn't alter over time, it could be used in the horticultural trade to propagate dwarfing plants. That might be good in the case of citrus, meaning less pruning. There's already the ‘Flying Dragon' rootstock that causes dwarfing, but it has some problems, like being really slow growing.
Anyway, another cause of undisciplined broom growth is phytoplasma infection. These are bacteria-like organisms that were only discovered in the 1960s and cannot be cultured. This makes it hard to work with them. They can cause debilitating weaknesses in plants besides the broom growth. To complicate the culturing issue, it has a conspirator, which needs to be understood, as well. The phytoplasma is spread by an insect, the same way the Liberibacter bacteria is spread by Asian Citrus Psyllid, causing HLB. So it's necessary to know the biologies of two actors, plus how they interact. The problem of HLB, again.
In India and Iran, there is a crippling disease of lime caused by a phytoplasma that is spread by leafhoppers. You need the specific leafhopper to spread the phytoplasma to spread the disease. No leafhopper and it's unlikely that even if the phytoplasma were here, that it would get spread.
The internet is democratizing. It spreads all kinds of information equally to everyone. So an image comes across my computer, asking if I think the wild growth on the ‘Pixie' mandarin is this WBDL (Witches' Broom Disease of Lime). I don't think so, but you never know what is going to pop up. I was thinking it's probably Sphaeropsis on a new track, and whatever, need to check it out.
And there it is. The wild growth. It fills about a third of the canopy of the little tree.
There's even this flattened stem. Truly weird.
But something does not look right. Is it the phytoplasma causing other types of growth? The leaves don't look right. Citrus leaves have a central vein, with lateral veins. The leaves on the affected part of the tree only have a single mid-vein.
This is not a citrus growth deformity. This is some other plant. Following the stem of the deformed branch to the base of the trunk, there's the “sucker”. And it doesn't look like citrus bark.
This is not even a citrus plant. At some point, a myoporum seedling started growing there. The leaves are similar enough to citrus, that the grower thought it was a citrus branch than had become infected. Well, the myoporum did become ‘infected'. With what, I'm not sure. It could easily have been Sphaeropsis since it is in the Ojai area. Anyway, “when it doubt, cut it out”.
And here is the neat little tree, a third of it gone now and the interior exposed to sun so that it will need to be whitewashed to protect it from sun damage. But, it's not WBDL. This time. It's important to keep looking for what nature and hitchhikers can bring us. It could be a whole lot more serious.