Garden Gate – October 2019

Dark-ringed spots

and straw-colored

foliage

58 GARDEN GATE | OCTOBER 2019 | ISSUE 149

B

oxwood blight is a fungal disease spreading

quickly across North America. Although we

don’t have as many boxwoods as the landscape

above in our Iowa test garden, gardeners who rely

heavily on boxwoods are really struggling. We

haven’t seen this disease’s devastation yet, but I

need to plan how to fi ght it when it comes.

WHAT IS BOXWOOD BLIGHT? Boxwood blight

is caused by the pathogen Calonectria pseudonavicu-

latum, and will also affect other shade-loving plants

like sweet box and pachysandra. The photos below

show the progression of symptoms: First you’ll notice

circular lesions with dark brown edges on the leaves

and black streaking on the stems. As the disease

spreads, foliage turns straw-colored, then falls off.

Sometimes this can be confused with volutella blight

or winter burn; a tissue test at a plant disease diag-

nostic clinic can confi rm boxwood blight.

Boxwood blight is spread by contact with infected

plants from nurseries, tools, clothing and even greens

in holiday decorations, such as a wreath or center-

piece. Spores can persist in the soil for up to 5 years,

so even a new boxwood planted where an infected

plant was removed can become diseased. There is

no cure, so you will need to either begin treatment

if you have too many plants to lose or remove the

entire plant.

WHAT TO DO ABOUT BOXWOOD BLIGHT If

the infection is in the early stages, cut off all affected

branches and clean up debris off the ground. Double

bag the trimmings and dispose of them in the trash,

burn (where permitted), or bury them at least 2 feet

deep in the ground. Don’t compost infected foliage.

You may be able to keep boxwood blight at

bay on remaining unaffected foliage by spraying a

chlorothalonil-containing fungicide every 7 to 14 days

during the growing season when temperatures are

above 60 degrees F. Reapply if it rains—the fungus

thrives in warm, humid weather. Use this fungicide

carefully according to directions, as it is toxic to fi sh.

Botanical

Names

Boxwood

Buxus spp.

and hybrids

Littleleaf boxwood

Buxus microphylla

Pachysandra

Pachysandra

spp. and hybrids

Sweet box

Sarcococca

spp. and hybrids

PHOTOS: Courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org (ringed spots and black streaks); courtesy of Adria Bordas, Virginia Polytechnic Institute and State University, Bugwood.org (boxwood blight progressio

n)

Signs of Boxwood Blight

Black streaks

on the stem

Starting with

interior leaves, the

plant becomes

defoliated

What To Do About

Boxwood Blight

GG14958_59_FTTG.indd 58GG14958_59_FTTG.indd 58 6/26/2019 11:26:25 AM6/26/2019 11:26:25 AM

Dark-ringedspots

andstraw-colored

foliage

58 GARDEN GATE | OCTOBER 2019 | ISSUE 149

B

oxwoodblightis a fungaldiseasespreading

quicklyacrossNorthAmerica.Althoughwe

don’thaveasmanyboxwoodsasthelandscape

aboveinourIowatestgarden,gardenerswhorely

heavilyonboxwoodsarereallystruggling.We

haven’tseenthisdisease’sdevastationyet,butI

needtoplanhowtofightit whenit comes.

WHATISBOXWOODBLIGHT? Boxwoodblight

is causedbythepathogenCalonectriapseudonavicu-

latum, andwillalsoaffectothershade-lovingplants

likesweetboxandpachysandra.Thephotosbelow

showtheprogressionofsymptoms:Firstyou’llnotice

circularlesionswithdarkbrownedgesontheleaves

andblackstreakingonthestems.Asthedisease

spreads,foliageturnsstraw-colored,thenfallsoff.

Sometimesthiscanbeconfusedwithvolutellablight

orwinterburn;a tissuetestata plantdiseasediag-

nosticcliniccanconfirmboxwoodblight.

Boxwoodblightis spreadbycontactwithinfected

plantsfromnurseries,tools,clothingandevengreens

inholidaydecorations, such as a wreath or center-

piece.Sporescanpersist in the soil for up to 5 years,

soevena newboxwood planted where an infected

plantwasremovedcanbecomediseased.Thereis

nocure,soyouwillneedtoeitherbegintreatment

if youhavetoomanyplantstoloseorremovethe

entireplant.

WHATTODOABOUTBOXWOODBLIGHT If

theinfectionis intheearlystages,cutoffallaffected

branchesandcleanupdebrisofftheground.Double

bagthetrimmingsanddisposeoftheminthetrash,

burn(wherepermitted),orburythematleast2 feet

deepintheground.Don’tcompostinfectedfoliage.

Youmaybeabletokeepboxwoodblightat

bayonremainingunaffectedfoliagebysprayinga

chlorothalonil-containingfungicideevery7 to 14 days

duringthegrowingseasonwhentemperaturesare

above 60 degreesF. Reapplyif it rains—thefungus

thrivesinwarm,humidweather.Usethisfungicide

carefullyaccordingtodirections,asit is toxictofish.

Botanical
Names

Boxwood
Buxusspp.
and hybrids
Littleleafboxwood
Buxusmicrophylla
Pachysandra
Pachysandra
spp. andhybrids
Sweet box
Sarcococca
spp. andhybrids

PHOTOS: Courtesy of Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org (ringed spots and black streaks); courtesy of Adria Bordas, Virginia Polytechnic Institute and State University, Bugwood.org (boxwood blight progressio

n)

SignsofBoxwoodBlight

Blackstreaks

onthestem

Starting with

interior leaves, the

plant becomes

defoliated

What To Do About

Boxwood Blight