Black Root RotFrank Hay and David H. GentThe fungus-like organismPhytophthora citricola causes a crown androot rot of hop referred to as black rootrot. The disease tends to be most damag<strong>in</strong>gto hop plants <strong>in</strong> poorly dra<strong>in</strong>ed soils andareas with high water tables. Certa<strong>in</strong>Cluster varieties such as Cluster types E-2and L-8 are particularly susceptible. Thepathogen has a relatively broad host rangethat <strong>in</strong>cludes cherry, fir trees, raspberry,strawberry, and walnut.SymptomsInfected roots and crowns have acharacteristic water-soaked and blackenedappearance with a dist<strong>in</strong>ct boundary betweendiseased and healthy tissue (Fig. 3).Infection can spread from the crown <strong>for</strong> several<strong>in</strong>ches up the base of the b<strong>in</strong>e. In severecases, leaves become yellow and b<strong>in</strong>es wiltrapidly dur<strong>in</strong>g warm weather or when plantsbecome moisture-stressed. Young plantsirrigated heavily to encourage production<strong>in</strong> the first year can wilt later <strong>in</strong> the seasonas a result of black root rot. As the diseaseprogresses, leaves turn black and rema<strong>in</strong> attachedto the b<strong>in</strong>e. Severely <strong>in</strong>fected plantsare weakened and may die dur<strong>in</strong>g w<strong>in</strong>teror the follow<strong>in</strong>g spr<strong>in</strong>g. Affected plantsoften are found <strong>in</strong> areas of hop yards withpoor dra<strong>in</strong>age. Wilt<strong>in</strong>g symptoms causedby black root rot can be mistaken <strong>for</strong> Verticilliumwilt, Fusarium canker, or damagecaused by Cali<strong>for</strong>nia prionus beetle.Disease CycleThe black root rot pathogen survives<strong>in</strong> soil as dormant sexual spores (oospores),which can survive 18 months or more. Inthe presence of free water and host roots,oospores or the asexual spores (sporangia)germ<strong>in</strong>ate and <strong>in</strong>fect the plant directly ormay release motile spores (zoospores) thatare attracted to compounds released fromhost roots (e.g., ethanol and certa<strong>in</strong> am<strong>in</strong>oacids and sugars). The motile zoospores settleon roots and later produce mycelia that<strong>in</strong>fect and grow through the host tissues.<strong>Management</strong>Growers should avoid establish<strong>in</strong>ghop yards <strong>in</strong> areas with poor water dra<strong>in</strong>age,especially with highly susceptible varietiessuch as Cluster types E-2 and L-8. ClusterL-1 and Galena are considered partiallyresistant, while Brewers Gold, Bullion,Cascade, Columbia, Comet, Eroica, Fuggle,Hallertauer, Nugget, Olympic, Tettnanger,and Willamette reportedly are highly resistantto black root rot. Reduc<strong>in</strong>g cultivation andavoid<strong>in</strong>g <strong>in</strong>jury to crowns and roots canprovide some reduction <strong>in</strong> disease s<strong>in</strong>ce<strong>in</strong>fection is favored by wounds. Certa<strong>in</strong>phosphorous acid fungicides are registered<strong>for</strong> control of black root rot, but theirefficacy has not been reported. Phenylamidefungicides (i.e., various <strong>for</strong>mulations ofRidomil) applied <strong>for</strong> control of downymildew may provide some control, althoughthese products are not registered specifically<strong>for</strong> control of black root rot.At-A-Glance:BlackRoot Rot◆◆Plant resistantvarieties whenpossible.◆◆Avoid poorlydra<strong>in</strong>ed fieldsand excessiveirrigation.◆◆Avoid damag<strong>in</strong>groots dur<strong>in</strong>gcultivation.◆◆Phosphorousacid fungicidesand variousRidomil<strong>for</strong>mulations mayprovide somecontrol.9Figure 3. Extensive black discoloration caused by black root rot. Notice the dist<strong>in</strong>ctmarg<strong>in</strong> between healthy tissue and the black, diseased tissue. (R. A. Beatson)See the PacificNorthwest Plant Disease<strong>Management</strong> Handbookat http://plant-disease.ippc.orst.edu/ <strong>for</strong> acurrent list of registeredherbicides.
10At-A-Glance:DownyMildew◆◆Select the mostresistant varietythat is available<strong>for</strong> the <strong>in</strong>tendedmarket.◆◆Establish hopyard with diseasefreeplant<strong>in</strong>gmaterials.◆◆Thoroughlyremove all basalfoliage dur<strong>in</strong>gspr<strong>in</strong>g prun<strong>in</strong>g.◆◆Prune yards aslate as possiblewithout adverselyaffect<strong>in</strong>g yield.◆◆Strip leavesfrom b<strong>in</strong>esafter tra<strong>in</strong><strong>in</strong>gand removebasal foliagewith chemicaldesiccants.◆◆Applyappropriatefungicides dur<strong>in</strong>gthe first year ofproduction andwhen weather isfavorable to thedisease.◆◆Rotateand tank-mixfungicides to delaydevelopment ofresistance.Downy MildewDavid H. Gent and Dennis A. JohnsonDowny mildew is caused by thefungus-like organism Pseudoperonosporahumuli. It is one of the most importantdiseases of hop <strong>in</strong> the Pacific Northwestand worldwide. Yield and quality lossesfrom downy mildew vary depend<strong>in</strong>g onsusceptibility of the variety and tim<strong>in</strong>gof <strong>in</strong>fection, and may range from nondetectableto 100% crop loss if significantcone <strong>in</strong>fection or plant death from crownrot occurs.Figure 4. Basal spikes: Hop shootssystemically <strong>in</strong>fected with the downymildew pathogen. (D. H. Gent)Figure 5. Profuse sporulation on theunderside of a hop leaf appears darkpurple to black. (D. H. Gent)Figure 6. Infection of shoots after tra<strong>in</strong><strong>in</strong>g.Notice the yellow<strong>in</strong>g, stunt<strong>in</strong>g, and downcurl<strong>in</strong>gof the leaves. (D. H. Gent)Figure 7. Stunted lateral branches result<strong>in</strong>gfrom downy mildew. Production from thesebranches will be lost. (D. H. Gent)See the Pacific Northwest PlantDisease <strong>Management</strong> Handbookat http://plant-disease.ippc.orst.edu/ <strong>for</strong> a current list ofregistered herbicides <strong>for</strong> downymildew and other diseases.
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