- Author: Ben Faber
- Author: Craig Kallsen
- Author: Akif Eskalen
Dry Root Rot is a pretty fantastic disease symptom that is usually seen in lemon, but can be seen in orange, as well.
Craig Kallsen, UCCE Citrus Advisor in Kern Co. comments on a disease sample:
I have seen a lot of dry root rot over the years. It usually is something that damages or weakens the root system which allows a Fusarium species to colonize the rootstock. It is very common in older lemon groves that froze at some point in their past. Also common in groves that suffered a lot of gopher damage or where the wraps got too hot in the sun burning the bark and cambium. I have also seen it in cases of fertilizer or other soil-applied chemical burn. I have no doubt that graft incompatibility could do it too.
Akif Eskalen, UCCE Plant Pathology Specialist chimes in on a disease sample submitted:
As you can see from the attached picture there is a weird callus formation and symptoms of incompatibility at the graft union which I think is the primary cause of decline. We didn't observe any discoloration in the scion, however rootstock was completely discolored where we isolated Fusarium solani the causal fungus of Dry Root Rot. Dry root rot caused by either Fusarium solani and/or Fusarium spp. When there is a disconnection at the graft union, the phloem can not transfer enough carbohydrate to the rootstock to feed feeder roots. Fusarium fungal species are present in the soil and they can attack and easily colonize on starch depleted roots and cause DRR.
We still don't know what is causing the graft incompatibility on these plants. That needs to be investigated.
It's still not clear how and why citrus becomes affected.
- Author: Ben Faber
Polyphagous Shothole Borer which is an ambrosia beetle that normally feeds on dead trees is going after live trees - over 100 species including sycamore, alder and coast live oak. It also goes after avocado. The California Avocado Commission has sponsored the placement of traps that have lures for PSHB. The traps are near avocado orchards but also likely spots where they might show up, such as campsite where people would bring firewood that might be infested with the beetle. The map here shows where the traps are located and where PSHB have been trapped. There are also traps in Santa Barbara County which are not yet shown on this map.
- Author: Akif Eskalen
The Polyphagous Shot Hole Borer (PSHB), Euwallacea sp. #1, is an invasive beetle that carries three fungi: Fusarium euwallaceae, Graphium sp. , and Acremonium sp. The adult female tunnels galleries into a wide variety of host trees, where it lays its eggs and grows the fungi. The fungi cause the Fusarium Dieback (FD) disease, which interrupts the transport of water and nutrients in over 35 tree species that are suitable for beetle reproduction.
Once the beetle/fungal complex has killed the host tree, pregnant females fly in search of a new host.
A separate invasion was recently detected in commercial avocado groves and landscape trees in San Diego county. It has been determined that the damage has been caused by another closely related species of PSHB (Euwallacea sp. #2), carrying a new species of Fusarium and Graphium. The beetle in LA, Orange, Riverside, and San Bernardino Counties are morphologically indistinguishable, but genetically distinct from the beetle found in San Diego County.
Signs and Symptoms
Attack symptoms, a host tree's visible response to stress, vary among host species. Staining, sugary exudate (B), gumming, and/or frass may be noticeable before the tiny beetles (females are typically 1.8-2.5 mm long). Beneath or near these symptoms, you may also see the beetle's entry/exit holes, which are ~0.85 mm in diameter. The abdomen of the female beetle can sometimes be seen sticking out of the hole.
Sugary exudate on trunks or branches may indicate a PSHB attack (photos A-E). Note that exudate may be washed off after rain events and therefore may not always be present on a
heavily infested branch.
PSHB attacks hundreds of tree species, but it can only successfully lay its eggs and/or grow the fungi in certain hosts. These include: Avocado, Box elder, California sycamore, Coast live oak, White alder, Japanese maple, and Red willow. Visit eskalenlab.ucr.edu for the full list.
Fusarium dieback pathogens cause brown to black discoloration in infected wood. Scraping away bark over the entry/exit hole reveals dark staining around the gallery, and cross sections of cut branches show the extent of infection. Advanced infections eventually lead to branch dieback and death of the tree
How to report a suspect tree
Please report suspected tree infestations to UC Riverside (email@example.com).
Submit the following information:
•Contact information (name, city, phone number, email)
•Suspect tree species
•Description of suspect tree's location (and/or GPS coordinates)
•Description of suspect tree's symptoms
•Photos of suspect tree and close-up photos of symptoms (see examples)
Take photos of suspect trees from several distances. Include photos of:
1. the trunk or symptomatic branches;
2. the symptoms (close-up); and
3. the entry/exit hole, if visible, with a ballpoint pen for scale (remove exudate if necessary). If dieback is observed, take a picture of the entire tree.
- Author: Gary Bender
The polyphagous shothole borers (Euwallacea sp.) that spread fungal diseases (Fusarium sp. and possibly Graphium sp.) to susceptible trees in Los Angeles County have now been found in mid and northern Orange County and western San Bernardino County. Sick and dying trees are being cut down and shredded or chipped. A lot of different species of trees are affected, including avocado, box, elder, castor bean, coast live oak, Engelmann oak, sycamore, bigleaf maple, California bay laurel, white alder, olive, peach persimmon, goldenrain, mimosa, liquid amber and wisteria vine.
Why is this important to growers in San Diego, Riverside, Ventura and Santa Barbara counties? Because growers in San Diego County (and probably other coastal counties) are being offered free shipping by the waste disposal companies of wood chips and free spreading of the mulch in their groves. What a deal!! But wait a minute!
The problem lies in that the material I have seen is either not composted or poorly composted, because it heats up in the grove after delivery and starts steaming. This means that freshly shredded or chipped trees could very likely be spreading the borers right into their groves!
Growers should ask themselves “why are these trees in Los Angeles being cut down in the first place?” Trees are being pruned and cut down for a variety of reasons, but now that we have a new pest for which there is no control, we have to be very cautious about what we bring into our groves. Other problems that could be brought into groves include Phytophthora root rot and trunk cankers, oak root fungus, Dothierella cankers, and Asian citrus psyllid.
Growers should insist that only correctly composted mulch be brought into their groves. During the composting process the piles should be turned at least five times to allow the material on the outside of the pile to be turned into the middle for correct heating of the entire pile.
- Author: Akif Eskalen
This is an update on our recent findings on PSHB/Fusarium dieback. As of October 4, 2013, PSHB/Fusarium dieback was detected in Glendora in northern Los Angeles county and Laguna Niguel in southern Orange County. The infestation in Laguna Niguel Regional park appears to have arrived there within the last week. We inspected the South coast research station last week, and it was still negative. Please find attached recent distribution map.