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QUESTION: Why
are leaves falling from live oak trees just as if it is
spring? Could it beoak wilt? This started before the heavy rains
of July, 2002, so it might be drought related. On every tree,
thebottoms of the leaves are covered with golden spots with tiny
projections. Is this an insect or disease and what can be done?
ANSWER: THIS
RESPONSE FROM MARK BLACK, Plant Pathologist in Uvalde:I
would need to have a sample to examine under the microscope to
be sure,but this looks like an oak rust disease. Jerral Johnson
was doing some work on it just before he retired to see if any
alternate hosts could befound, but I never heard anything conclusive.
I see a rust on Lacey oak in the Uvalde area periodically, but not every year. The rusts are
often veryhost specific, but this one might go to several local
oaks.
The infection period was probably back in the spring soon after this
yearsleaves emerged, during a wet period. The recent rains might
haveincreased the sporulation potential of lesions, but probably
did not affect thenumber of leaves or trees affected. The live
cycle is not known, so itcould have some herbaceous weed as the
alternate host and thisweed could have done well in last winter.
The spores being produced on live oakprobably will not re-infect
live oak, but only the alternate host(s). Systemic fungicide applications
at this time of the season and theinfection cycle would probably
have little if any noticeable effect. Theaffected trees may put
on new leaves with the recent soil moisture; aN-fertilizer application
next spring may be in order to stimulate newgrowth and help build
food reserves in the tree.
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Then from Larry Barnes, Plant Pathologist
in College Station:The images suggest
to me the possibility of leaf rust. Does theyellow substance come
off on your finger if you rub the undersurfaceof the leaf? If
so, I suspect leaf rust. this is not usually a problem ofignificance
and although control could be achieved by fungicides,most cannot
achieve the coverage needed to make fungicideapplication worthwhile.
The time of the year is marginal for efforts tocontrol leaf diseases
also. Maybe Mike or Mark will have a bettercomment. Rust could
be easily comfirmed if you send a sample-but "yellow dust
on the finger test" is also pretty good.
Larry Barnes
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Jerral Johnson, retired Plant Pathologist
in Arkansas replies:
This looks like rust. I have observed it in
your surroundingcounties and it can cause severe loss of foliage.
It does not occur on aregular basis. It was identified as a pine
cone rust. Oak is thealternate host. The projections are the telia
horns which areproducing telia spores. I do not know of any control
on the oak. Itoccurs so seldom that a regular spray would not
be feasible. I observed that it appeared to have a limited time
for infection. By the end of the year the leaves would be replaced.
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