DISEASE UPDATE : Springtime Preparation for Common Diseases of Landscape Trees and Shrubs
By Nick Brazee , UMass Extension Plant Pathologist
Late winter is a good time to think about preventative treatments for a variety of landscape tree and shrub diseases . Dormant season management can reduce or eliminate the overwintering inoculum source that initiates infections on new growth in the spring . In addition , cold temperatures prevent most pathogens from growing , limiting the risk of spread at this time . Pervasive and damaging diseases like fire blight , fungal stem and branch cankering , black knot of Prunus , and foliar anthracnose are examples where preventative care can make a difference in successful management .
Fire Blight : The bacterium Erwinia amylovora is responsible for fire blight . While the potential host range for this pathogen spans many rosaceous trees and shrubs , apple and pear are the primary landscape hosts . Preventative management should focus on the removal of dead branches and last season ’ s fruit . Branches , stems and fruit diseased by E . amylovora will often ( but not always ) appear black in color ( as if blackened by fire , hence the name ). Fire blight infections mostly occur in May when trees are in bloom . However , after the damage takes place , opportunistic cankering pathogens ( described below ) can often establish on diseased tissues , furthering the dieback . For proper identification of fire blight , samples should be collected early in the growing season . Pruning and removal of blighted stems and fruit when temperatures are below 40o F is ideal . Remove approximately eight inches of the branch away from the transition zone between diseased and healthy tissue ( if possible ) or remove the entire branch at the collar .
Stem Cankering / Blight : Numerous fungal pathogens that are widespread in the environment are responsible for stem cankering diseases on landscape trees and shrubs . A few of the most prominent and destructive include Botryosphaeria s . l ., Phomopsis , Cytospora and Nectria . These fungi can overwinter within active cankers on live branches and on dead material lingering in the canopy . Again , when temperatures are below 40 º F , these fungi will be dormant and there is little risk of spread . Removing infected and dead parts substantially reduces the overwintering inoculum these fungi use to produce infective spores when temperatures warm . Newly flushed shoots and leaves are highly susceptible to infection in the spring season before the protective bark is fully formed . Just as with fire blight , prune several inches away from the transition zone between diseased and healthy tissue to ensure removal of the pathogen . If trees and shrubs with persistent stem cankering infections are growing near lawns that receive overhead irrigation , redirect the spray away from these plants . While the supplemental water is helpful during prolonged dry periods , irrigation that regularly wets the canopy provides the necessary moisture required by cankering pathogens to initiate new infections and readily spread throughout the canopy once established . If drought stress is a threat , drip irrigation or soaker hoses should be utilized instead .
Black Knot : The fungal pathogen Apiosporina morbosa is responsible for black knot . All species in genus Prunus are susceptible , but cherry and plum typically suffer from the worst infections in this region . The dormant season is the best time to scout for black knot before the cankers and galls become concealed by foliage . The early stages of black knot can be difficult to identify without very careful scouting . The cankers appear as circular to oval-shaped swellings on small diameter stems . These cankers may quickly girdle infected stems or the stems remain alive and perennial cankers develop . When perennial cankers are present in the canopy , the symptoms appear as enlarged , splitting / cracking , black-colored galls . The galls are a mixture of fungal and host tissue and these infected stems and branches can persist for many years . However , when the disease is widespread throughout the canopy , decline is inevitable . The pathogen thrives on trees in shaded settings , so ensure that cherries and plums receive full sun . Prune out interior canopy stems and branches that are heavily shaded , ensuring that all branches are receiving ample direct sunlight . Removing interior canopy stems and branches will also improve air flow , limiting the time that moisture persists . Black knot can co-occur with brown rot ( caused by Monilinia spp .). Once the two pathogens become established , it ’ s very challenging to maintain a tree with any aesthetic value or sufficient fruit production . Prune infected branches with knots or one-year-old stems with evidence of cankering ( discolored or sunken bark , oozing sap ) at least four inches away from visible symptoms . This sanitation pruning can coincide with structural pruning to improve light and air flow . Dispose of the debris off site to ensure the pathogen doesn ’ t sporulate from stems and branches piled nearby .
Foliar Diseases of Deciduous Hardwoods : Late winter and early spring interventions aid in the management of leaf spot diseases such as apple scab , black spot on rose , tubakia leaf blotch of oak , tar spot of Norway maple , and foliar anthracnose diseases . Foliar disease fungi overwinter in discarded leaves nearby or by lingering in the canopy . Some of these pathogens also overwinter underneath bud scales or within dead stems .
Reduce immediate sources of inoculum by removal of leaves on the ground around susceptible trees and shrubs . A fall and spring removal may be required , especially when trees are in beds with dense shrubs
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MLP Newsline | Spring 2022 13