Elms die suddenly or gradually. In the acute form of disease first the
young leaves and then all the leaves wilt and wither so rapidly they curl,
dry, and fall while still green. Chronic symptoms are more gradual. Individual branches or "flags" appear yellow against the green of the rest of
the tree; sometimes all the foliage gradually turns yellow. Some trees
leaf out in spring with sparse, chlorotic foliage and a staghead appearance. A toxin secreted by the fungus is carried in the sap stream, and
when an infected twig or branch is cut across, the water-conducting vessels are black. The fungus lives in sapwood, producing spores in cracks
and in bark beetle galleries. When the bark beetles emerge from dead or
dying trees, they carry the spores along, thereby inoculating healthy twigs
as they feed at the crotches of leaf axils (Figure 7E). The disease is also
transmitted by natural root grafts when streets are planted so solidly to
elms their roots are in contact.
Control. A dormant spray with a heavy dose of DDT, 2% emulsion
for hydraulic sprayers and 12% for mist blowers, and a foliage spray 2
to 3 months later at half the dormant strength helps to control bark
beetles. The foliage spray also helps reduce elm leaf beetles which weaken
trees by their feeding and make them more liable to be used for breeding
by bark beetles. Dead trees should be removed or the bark treated before
beetles emerge in spring, usually before May first. Chemotherapy, treat-
ing the soil with a chemical to inactivate the toxin apparently has value
as a preventive measure. Oxyquinoline benzoate, at the rate of 2/3 ounce
per diameter inch of trunk, dissolved in water and applied to soil by
subsurface injection, offers enough promise so that it is being used by
some tree concerns for their clients. The search continues for resistant
MIMOSA WILT (Fusarium oxysporum f. perniciosum).
This disease has
killed thousands of mimosas in the Southeast. Leaves wilt, hang
down, dry, and drop, with death of the entire tree following in a month
or a year. The trunk has a brown ring of discolored sapwood. The fungus
is another soil pathogen, entering through the roots. Neither spraying
nor eradication of diseased trees stops the spread of mimosa wilt. Resistant
varieties have been developed and are now being propagated.
OAK WILT (Chalara quercina = Endoconidiophora fagacearum).
This disease bids fair to rival chestnut blight in its destructiveness. It is now
known in at least 19 states, a menace from Nebraska and Minnesota south
to Arkansas, from Pennsylvania west to Missouri. First symptoms are a
slight crinkling and paling of leaves, followed by progressive wilting,
bronzing, and browning of leaf blades from margin toward midrib, and
defoliation progressively downward and inward. Red oaks never recover;
they may be killed in 4 to 8 weeks.