BP_39_W Apple Scab of Crabapple
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Apple Scab of Flowering Crabapples Flowering crabapples add color and beauty to many home landscapes in Indiana. However, each year numerous flowering crabapples become diseased, resulting in yellow leaves and defoliation by early summer. The cause of this problem is a fungus disease known as apple scab. The same disease is also a common problem on eating apples, refer to BP-1 (Apple Scab in the Home Fruit Planting). With the use of resistant cultivars, or with a combination of cultural and chemical controls, apple scab can be minimized, providing a tree that continues to add beauty to the home landscape through summer and fall. Symptoms Brown to olive-green, roughly circular, fuzzy spots often originate along the veins of the leaves. In time, spots become black and velvety and develop fringed margins. Finally, leaves yellow and drop. Very susceptible cultivars may show a very rapid blackening and curling of leaf tissue. In a “bad” scab year it is not unusual for trees to suffer near complete defoliation by the end of June. Small, round, dark areas that become corky frequently appear on fruits. Cause Apple scab is caused by the fungus Venturia inaequalis. The fungus lives through the winter within infected leaves. In early spring, spores of the fungus are shot into the air when leaves become wet; spores are then carried by wind to the newly developing apple leaves and cause leaf and/or fruit infection. Once infection has occurred, a different kind of spore is produced; these “summer” spores are capable of causing further infections through- out summer and early fall. This cycle repeats itself annually. Remedies for Apple Scab Resistance: Resistance is the best, and most fool-proof, means of manag- ing scab of flowering crabapples. Many desirable cultivars of resistant flowering crabapples are available; see Table 1 for a listing of cultivars and their scab suscep- tibility. Cultivars are grouped into classes to help nurserymen and homeowners choose those crabapples that are less prone to apple scab. Cultivars in Class III are not recommended for future Figure 1 - The final stage–leaf yellowing. planting because of their extreme susceptibility to apple scab. Cultivars in both Classes I and II have sufficient resistance to be recommended for future planting. Resistance to Japanese beetle should also be considered when selecting crabapple cultivars; refer to ID-217 (Crabapples Resistant to Apple Scab and Japanese Beetle in Indiana). Cultural Practices: Since fallen leaves harbor the scab fungus, rake and destroy them before they become brittle and break into tiny fragments that are difficult, if not impossible, to rake. Prune crabapples in late winter to maintain an “open” tree. A well-pruned tree allows better air circulation, faster drying conditions, and provides for better penetration of spray materials. Chemical Practices: Crabapples that are suscep- tible to apple scab need to be sprayed each year on a regular schedule to prevent infection. The most critical time to apply fungi- cides is spring (April and May). Generally a minimum of 3 to 4 sprays are required for adequate control of apple scab. “The secret to good scab control is the timely application of fungicides during April and May.” BP-39-W
Transcript of BP_39_W Apple Scab of Crabapple
Apple Scab ofFlowering Crabapples
Flowering crabapples add color andbeauty to many home landscapes inIndiana. However, each year numerousflowering crabapples become diseased,resulting in yellow leaves and defoliationby early summer. The cause of thisproblem is a fungus disease known asapple scab. The same disease is also acommon problem on eating apples, referto BP-1 (Apple Scab in the Home FruitPlanting). With the use of resistantcultivars, or with a combination ofcultural and chemical controls, apple scabcan be minimized, providing a tree thatcontinues to add beauty to the homelandscape through summer and fall.
SymptomsBrown to olive-green, roughly
circular, fuzzy spots often originate alongthe veins of the leaves. In time, spotsbecome black and velvety and develop
fringed margins. Finally, leavesyellow and drop. Very susceptiblecultivars may show a very rapidblackening and curling of leaftissue. In a “bad” scab year it isnot unusual for trees to suffer nearcomplete defoliation by the end ofJune. Small, round, dark areas thatbecome corky frequently appearon fruits.
CauseApple scab is caused by the
fungus Venturia inaequalis. Thefungus lives through the winterwithin infected leaves. In earlyspring, spores of the fungus areshot into the air when leavesbecome wet; spores are thencarried by wind to the newlydeveloping apple leaves and causeleaf and/or fruit infection. Onceinfection has occurred, a differentkind of spore is produced; these“summer” spores are capable ofcausing further infections through-out summer and early fall. Thiscycle repeats itself annually.
Remedies for AppleScabResistance:
Resistance is the best, andmost fool-proof, means of manag-ing scab of flowering crabapples.Many desirable cultivars ofresistant flowering crabapples areavailable; see Table 1 for a listingof cultivars and their scab suscep-tibility. Cultivars are grouped intoclasses to help nurserymen andhomeowners choose thosecrabapples that are less prone toapple scab. Cultivars in Class IIIare not recommended for future
Figure 1 - The final stage–leaf yellowing.
planting because of their extremesusceptibility to apple scab.Cultivars in both Classes I and IIhave sufficient resistance to berecommended for future planting.Resistance to Japanese beetleshould also be considered whenselecting crabapple cultivars; referto ID-217 (Crabapples Resistantto Apple Scab and JapaneseBeetle in Indiana).Cultural Practices:
Since fallen leaves harbor thescab fungus, rake and destroythem before they become brittleand break into tiny fragments thatare difficult, if not impossible, torake. Prune crabapples in latewinter to maintain an “open” tree.A well-pruned tree allows betterair circulation, faster dryingconditions, and provides for betterpenetration of spray materials.Chemical Practices:
Crabapples that are suscep-tible to apple scab need to besprayed each year on a regularschedule to prevent infection. Themost critical time to apply fungi-cides is spring (April and May).Generally a minimum of 3 to 4sprays are required for adequatecontrol of apple scab.
“The secret togood scabcontrol is thetimelyapplication offungicidesduring April andMay.”
BP-39-W
Crabapples should be sprayed ona regular schedule starting just asflower buds begin to show pink -BEFORE BLOOM ! Continuespraying on a 7-to-10 day schedule (7days during wet weather, 10 days ifdry) until dry weather prevails.
NOTE: Fungicides act as aprotective coat of “paint” on the leafsurface; where possible, apply fungi-cides just before aprolonged wetperiod occurs, notafter.
Fungicideseffective in control-ling apple scabinclude: captan,sold as Captan andas primary fungi-cide in manygeneral-purposegarden pesticides;chlorothalonil, soldas Daconil 2787,Fung-onil., etc.;thiophanate-methyl, sold as
Class III(High Susceptibility)
AdamsBrandywineCandied AppleIndian MagicIndian SummerProfusionRobinsonSnowdriftVelvet PillarWhite CandleWhite Cascade
Class I(High Resistance)
Ann E.BasakatongBob WhiteJackJapanese FloweringLouisaOrmiston RoyPrairie MaidPrairifireRedbudRed JewelSargentSilver Moon*Sinai FireSugar TymeTeaWhite Angel
Class II(Moderate Resistance)
CanaryCandymint SargentCenturionDavidDonald WymanDoubloonsHarvest GoldJewelberryLisetMadonna*Mary PotterMolten LavaSelkirkSentinelSilver Drift
* Not recommended for planting because of susceptibility to fire blight.
Table 1. Classes of crabapple cultivars based on their resistance to apple scab.
Figure 3 - Early leaf symptoms. Olivegreen, circular, fuzzy spots appear inearly spring.
Halt, Cleary’s 3336, Domain, Fungo,etc.; myclobutanil, sold asImmunox; propiconazole, sold asBanner MAXX; and fenarimol, soldas Rubigan. Not all of these chemi-cals will be readily available tohomeowners; some are primarily foruse by commercial nursery growersand landscape professionals. Fungi-
cides vary in their formulation andpercent active ingredient. Follow alllabel directions regarding amounts of
pesticide to use, methods of application,and safety warnings. Fungicides are notharmful to honeybees and may beapplied during bloom.
There are also a number of speciallyformulated, general purpose gardenpesticides that contain the above listedfungicides and one or more insecticides,such as Captan Garden Spray, FruitGuard, Liquid Fruit Tree Spray, etc.
Check the label of such products tobe sure they contain one of therecommended fungicides; also, ifyou use a general purpose pesticidecontaining an insecticide, DO NOTspray during bloom; insecticides aretoxic to honeybees.
References to products in thispublication are not intended to be anendorsement to the exclusion ofothers which may be similar. Per-sons using such products assumeresponsibility for their use in accor-dance with current label directionsof the manufacturer.
Figure 2 - Apple scab fruit symptoms.
The first and most important stepbefore managing a tree disease is toaccurately diagnose the problem.With an inaccurate diagnosis, moreharm than good could be done, notto mention the wasting of both timeand money.
This publication is just one ofseveral available online fromPurdue Extension that addressesdiseases found on landscape trees inIndiana. If your tree does not havesymptoms similar to those de-scribed in this publication, pleasecheck the others.
Also, for more detailed photo-graphs of disease symptoms,consider purchasing Common TreeDiseases of Indiana (BP-63). Itpresents information about the sixmost common tree diseases seen inIndiana. It is available from thePurdue Extension Media Distribu-tion Center. The publication is $5and can be ordered by calling 1-888-EXT-INFO.
If you are still in doubt as to thecause of the problem, consult aprofessional such as the ExtensionEducators at your local PurdueUniversity Cooperative ExtensionService office or Purdue University’sPlant Pest and Diagnostic Laboratory(P&PDL).
To submit a plant sample to theP&PDL for diagnosis, obtain a samplesubmission form from your localPurdue Extension office, from theP&PDL office (1-888-EXT-INFO), orfrom the P&PDL Web pagewww.ppdl.purdue.edu/. Detailedinstructions for submitting most typesof samples are included on the back ofthe forms.
Submit a sample that is representa-tive of the problem and shows thevarying degrees of symptoms. Sendseveral branches (even large ones)showing the symptoms and a detaileddescription of the problem and otheruseful information about the site, theage of the tree or shrub, and the dateof planting. Photographs are veryhelpful.
Send the sample and submis-sion form by first-class or over-night mail early in the week to:
Plant & Pest Diagnostic LaboratoryPurdue University1155 LSPSWest Lafayette, IN 47907-1155
REVISED 3/02It is the policy of the Purdue University Cooperative Extension Service, David C. Petritz, Director, that all persons shall have equal opportunity and
access to the programs and facilities without regard to race, color, sex, religion, national origin, age, marital status, parental status, sexual orientation,or disability. Purdue University is an Affirmative Action employer.
This material may be available in alternative formats.
1-888-EXT-INFO
