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Soil-Borne Disease Watch
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Root Rots & Wilts — Below-Ground Read
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This week's weather split is driving the two major soil pathogens in opposite directions:
drought is waking Armillaria in the Northeast while saturated soils are flaring Phytophthora in
the Mid-Atlantic and Southeast. All of them mimic drought stress above ground — and none
can be cured once the crown is talking.
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Northeast · Drought
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Armillaria root rot (shoestring root rot) — prolonged
drought pushes this ubiquitous fungus into aggressive attack on stressed oaks, maples, and
conifers, producing sudden mid-summer collapses and crown thinning that reads as drought.
Diagnostic: white, fan-shaped mycelial mats under the bark at the root collar, black
"shoestring" rhizomorphs in the soil, and honey-colored mushroom clusters at the
base once October rains arrive. No fungicide works; do not replant a killed tree's spot
with another woody species.
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Mid-Atl / SE · Wet
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Phytophthora root & crown rot — warm, saturated soil
for as little as 4–8 hours lets this water mold infect feeder roots, and
hurricane-season rain plus over-irrigation are setting it off on poorly drained sites
— rhododendron, azalea, dogwood, cherry, oak, and Taxus lead the list. Symptoms look
like drought (dull, wilting, red or purplish foliage) but the plant does not recover when
watered; look for darkened, gummy bark at the crown and reddish streaks in the inner bark.
Foliar symptoms lag severe root damage, so fungicides rarely rescue an infected tree.
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Midwest / NE · Maple
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Verticillium wilt — one-sided branch flagging and
premature fall color on maple, redbud, ash, smoke tree, and magnolia are at peak visibility
now; drought worsens it by blocking the tree's ability to wall off the fungus. Confirm
with olive-green to brown sapwood streaking in a cross-cut branch and a lab culture. No cure
— prune out dead wood, keep the tree watered and vigorous, and replant only with
non-hosts (oak, beech, sycamore, conifers).
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℞ Fall is the below-ground diagnostic window. Air-spade the root
collar on any declining tree and check for mycelial fans, crown cankers, and girdling roots
before you write a treatment. Fix the water first — correct drainage, pull irrigation off
trunks and crowns, and water at the dripline. For high-value, not-yet-symptomatic trees on
Phytophthora sites,
phosphite (potassium phosphonate)
drenches or trunk injections and
mefenoxam
soil applications are protective, not curative. For Armillaria and Verticillium, vigor is the
only treatment: irrigate through drought, mulch, avoid root and collar wounds, and mark
declining trees for October mushroom checks rather than removing them mid-season.
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