Photograph by Shari Kaplan
Dog-sized pumpkins are the norm around Halloween, but with the right care, these squashes can grow much larger.
'Sudden death' of oak trees becomes a complex plague
By Tony Tomeo
Recently, much of my work has involved the epidemic of "sudden death" of oaks. Although the name is simple, the disease is complex and still not completely understood by those who have studied it since it first appeared in Marin County several years ago.
When I first wrote about this disease, there were several theories about how and why it is so easily transmitted. One theory has been getting more attention from entomologists and pathologists, but still has not suggested a cure or means to slow the disease's spread.
Sudden death of oaks is caused by ambrosia fungus, which is spread by the oak ambrosia beetle. The beetles bore into the trunks of mature oaks and infest their galleries (bored tunnels) with spores of the fungus. The fungus grows profusely, providing sustenance to the beetles. The beetles actually eat only the mycelia of the fungus, and not the wood into which they bore.
Besides providing an ideal environment for the fungus, the galleries are also where the beetles lay eggs for the next generation. When second generation beetles leave the parents' galleries, they grab some spores on their way out to take to their new home in another oak tree, starting the process all over again.
Back at the parent beetles' galleries, the fungus continues to proliferate, eventually clogging the vascular system of the tree and killing it. Beetles do not normally live in small populations, but establish entire subdivisions in a single tree so that there are hundreds or thousands of galleries functioning at the same time.
With so much ambrosia fungus, death is so sudden that the leaves don't have time to fall. Trees that die in this manner are distinct from trees that die a normal, slow death, losing their leaves in the process. Unfortunately, the beetles are also very fast and a new generation has already moved on before infected trees die.
Infestation by the oak ambrosia beetle is usually, but not always, preceded by infestation of oak bark beetles, which can kill trees as they excavate around the trunk just under the bark. If they manage to get all the way around the trunk, the tree dies; but. normally, trees survive repeated attacks. Unfortunately, the stress caused by an attack is just the type of thing that attracts the ambrosia beetles.
Initially, it was thought that the oak bark beetle was the primary pathogen, followed by the oak ambrosia beetle. However, the attack on so many seemingly healthy trees could not be explained. Now the primary pathogen is believed to be a root rot fungus (genus phytophthora) that has yet to be isolated and identified.
The stress caused by this fungus is not obvious to people, but very attractive to beetles. Infection of root rot would also explain why shoots that sprout from the roots of a few of the killed trees die immediately after they emerge.
Anyone with coast live oaks, tanbark oaks or black oaks in their garden should be aware of the sudden death syndrome and how to recognize the symptoms. It is also important to know what to do about it and how to minimize the risk of infection. These topics will be discussed next week.
Plant of the Week: Pumpkin
Everyone is aware of pumpkins this time of year, but only those who thought of them last spring now have them growing in their gardens. Those who want to grow pumpkins next year need to give them an early start by planting seeds in May or early June.
The vines prefer direct sun exposure, regular irrigation and rich, well-drained soil. They can sprawl amongst other vegetable plants in the garden, but produce the best fruit if given their own space. Plants should be spaced according to variety, but are most often in groups several feet apart. They perform best if planted from seed rather than as small plants. (Seeds grow faster than small plants that need time to recover from transplanting.)
There are many varieties of pumpkins, so selection is important. Those that are grown to produce large fruit should be "culled," which involves selection of three or four of the best fruit closer to the roots and removal of all others before they become larger than about the size of a baseball, as well as flowers so that no other fruit develops. The largest pumpkins are produced by removal of all but one fruit--likely the procedure used by Cinderella's mechanic.
"Pie" pumpkins, grown for their flesh, do not require such selective culling. At least one variety is grown for the nut-like seeds, while others are grown for their ornamental, but inedible, 3-inch-wide fruit. Although jack-o-lantern and white pumpkins are not as well-flavored as pie pumpkins, they can also be used for pies or other recipes.
Horticulturist Tony Tomeo can be contacted at 408.358.2574.
Saratoga, California Since 1955
