submitted by Damon Waitt [Email address: waittd #AT# swbell.net - replace #AT# with @ ]
A recent news story in the AAS raises the question whether
Allandale residents should be concerned about oak wilt in our neighborhood.
Unlike Lakeway and other areas in the Texas Hill Country, the Allandale urban
forest is not characterized by dense stands of live oak with interconnected
root systems that facilitate the spread of the oak wilt fungus. That’s the good
news. However, oak wilt can also be transmitted by sap feeding beetles carrying
spores from infected red oaks to improperly pruned or wounded live oaks.
Following are some common question and answers about oak wilt if you are not
familiar with this disease that is responsible for killing over a million oaks
in central Texas.
What is oak wilt?
Oak wilt is an infectious disease caused by the fungus Ceratocystis fagacearum, which invades
and disables the water-conducting system of certain oak trees.
Where is oak wilt a problem?
Oak wilt has been found in over 60
counties and in almost every city in Central Texas. It can be a problem
wherever live oaks tend to be the predominate tree. It does not matter whether
they are transplanted or naturally grown.
White oaks are the least
susceptible. They generally survive for a number of years with the disease.
Common White Oaks include Post, Bur, Chinkapin, and Monterrey oak.
Red oaks are the most susceptible.
They typically die within 2-4 weeks of symptom appearance. Common red oaks are
Spanish, Texas, Shumard, Pin, and Blackjack oak.
Live oaks die in the greatest
numbers, most often in expanding areas called Oak Wilt Centers.
How can I tell if my trees have oak wilt?
Live Oak trees die within 6-12
months. Adjacent trees begin to drop their leaves and die a few months later.
Live Oaks also show veinal necrosis where the area around the leaf vein turns
brown but the rest of leaf is still green.
Red Oak symptoms are less distinct.
Leaves turn pale green then brown usually remaining attached for a period of
time. This can begin on one branch and quickly engulf the entire tree. Trees
generally die within 4-6 weeks.
How does oak wilt spread?
Oak wilt is spread by two methods,
root transmission and by sap-feeding (Nitidulid) beetles. The fungus spreads
from tree to tree through connected roots, either by root grafts or through a
common root system. When oak wilt does
infect a red oak species these trees under the right conditions can form what
is called a fungal mat. The fungal mats
form underneath the bark and produce millions of spores. Fungal mats have a
very sweet smell like ripe fruit which attracts Nitidulid beetles. The beetles
feed and breed on fungal mats and pick up spores of the oak wilt fungus on
their bodies. If a contaminated beetle lands on a fresh wound on a healthy oak,
that tree can become infected with oak wilt.
How can I prevent my trees from getting oak wilt?
New oak wilt centers are started
when a contaminated beetle finds a fresh wound on a healthy oak tree. Insect
populations in general increase during mild spring like weather and mild spring
like weather is when fungal mats are most likely to form. So the most likely
time a contaminated beetle will find your tree is when the weather is mild. The
best time to prune is when a contaminated beetle is least likely to find your
tree. The heat of summer and the cold of winter are when the beetle populations
are the lowest and fungal mats are least likely to form. Therefore that is the
best time to prune to prevent against an oak wilt infection.
The Texas Forest Service recommends
that you always paint pruning cuts as soon as the cuts are made. When pruning
cuts are made during the proper times of the year there still maybe a few
contaminated Nitidulid beetles in the area. If the pruning cuts are painted
with a pruning paint or latex paint this will prevent these sap feeding
Nitidulid beetles from feeding and infecting your oak tree with the oak wilt
Senior Botanist, Lyndon Baines Johnson Wildflower Center