Field Guide for Integrated Pest Management in Hops

22At-A-GlanceRed Crown Rot◆◆Select and plantonly high-qualityplanting materials.◆◆Avoid woundingcrowns duringspring pruning.◆◆Maintain plantvigor by avoidingearly harvests,maintaining basalfoliage, andavoiding soilappliedherbicidesthat reduce rootdevelopment.◆◆Avoid replantingin the hole leftby removing adiseased plant.◆◆Fumigation canimprove plant vigorand yield, but hasnot been adoptedwidely in Australiawhere this diseaseprimarily occurs.Figure 32. Reddish-browndecay and dry rot of acrown affected by redcrown rot. (D. R. Smith)Red Crown RotDavid H. GentRed crown rot has been described onhop plants in Australia and Oregon. In Australiathe disease was attributed to a fungusdescribed as a Phacidiopycnis sp. The namingof this fungus was in flux, and the propername of this organism is now thought to bePhomopsis tuberivora. Data from Australiaindicate affected plants may suffer yield lossesof up to 20%. In Oregon, plants have beenkilled by red crown rot and yield losses appearto be higher than 20% in some instances.SymptomsThe pith tissue of affected roots andcrowns is orange to red, which developsinto a dry rot of the root (Figs. 32-33) witha distinct boundary between diseased andhealthy tissue (Fig. 33). Roots and crowns ofapparently healthy plants also may have thisappearance, but the degree and severity of rotis more pronounced in diseased plants. In theadvanced stages of the disease, entire crownsare destroyed, leading to weak, uneven shootgrowth and yellowing of lower leaves (Fig.34). Bines on severely affected plants oftenfail to reach the top wire and have limiteddevelopment of lateral branches. Severelyaffected plants can be killed. Affected plantstend to be aggregated in roughly circularpatches, although in some young hop yardsdiseased plants may be more generallyscattered across a yard.Disease CycleThe only data available on the diseasecycle of red crown rot are from researchconducted in Victoria, Australia. In thatenvironment, the disease was thought tobe associated with planting poor qualityFigure 33. Reddish-brown rot of a younghop root caused by red crown rot. Noticethe distinct margin between diseased andhealthy pith tissues. (D. H. Gent)Figure 34. Weak growth of bines andplant death caused by severe red crownrot. Notice that affected plants areaggregated in this yard. (D. R. Smith)rootstock, injury to crowns during springmowing of shoots (slashing), and culturalpractices that reduced plant vigor, such asearly harvest and leaving insufficient foliageon plants after harvest. The causal organismcan be recovered from soil, plant debris,and healthy crowns. The host range of thepathogen also includes alfalfa, beet, potato,and several trees and woody ornamentals.The fungus is a weak pathogen and diseasesymptoms rarely develop on these hosts.ManagementControl measures for red crown rothave not been investigated or validated inthe Pacific Northwest; the disease currentlyappears to cause economic damage inrelatively few yards. Red crown rot hasbeen managed successfully in Australiathrough a combination of careful selectionof high quality, disease-free plantingmaterials, avoidance of crown woundingduring spring pruning, and culturalpractices that maintain plant vigor. Othermanagement recommendations promotedin Australia include removing diseasedplants and avoiding replanting in the holeleft by removing a diseased plant. Effortsshould be made to improve plant vigorby avoiding early harvests, maintaining asmuch foliage as possible after harvest tohelp plants increase carbohydrate reserves,and avoiding soil-applied herbicides thatreduce root development. Boron deficiencyhas been implicated in red crown rot inVictoria, although conclusive evidence ofa link between boron deficiency and thedisease is lacking. In Victoria, fumigationwith dazomet provided an approximate60% increase in yield in year one and 14%in year two. However, this practice has notbeen adopted in Australia due to the highcost of fumigation.