Plant Answers  >  Questions with Photos Index: Oak Leaf Rust (Why are leaves falling from Live Oak trees just as if it is spring.)
Questions with Photos Index

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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