Evaluation of Hydrangea macrophylla for Resistance to ...

1 Otis A. Floyd Nursery Research Center, Tennessee State University, McMinnville, TN, USAE valuation ofHydrangeamacrophyllafor Resistance to leaf -Spot DiseasesMargaretargaretT. Mmbagambaga1,Meeee-SookookKimim2,Lucasuc asMackasmielackasmiel1and YonghaoonghaoLii3 Authors addresses:1 Tennessee State University, School of Agriculture and Consumer Science, Otis A. Floyd NurseryResearch Center, 472 Cadillac Lane, McMinnville, TN 37110, USA;2 Department of Forestry, Environment, and Systems,Kookmin University, Seoul 136-702, Korea;3 Department of Plant Pathology and Ecology, Connecticut AgriculturalExperiment Station, New Haven, CT 06511, USA (correspondence to M. T. Mmbaga. E-mail: April 25, 2011; accepted September 22, 2011 Keywords:Corynespora cassiicola,Cercosporaspp.,Glomerella cingulata,Colletotrichum gloeosporioides,Phoma exigua,Myrothecium roridum,Botrytis cinerea, Hydrangea macrophylla , Resistance selectionsAbstractGarden Hydrangea ( Hydrangea macrophylla )isapopular ornamental plant that can be devastated byleaf- spot diseases .)

2 Information is needed to determinesusceptibility of commercial cultivars to leaf -spot dis-eases. To address this need, 88 cultivars ofH. mac-rophyllawere evaluated for their Resistance to leaf - spot diseases in full-shade (2007 2008), full-sun(2007 2008) and partial-shade (2009 2010) environ-ments in McMinnville, TN, USA. Ten cultivars[ Ami Pasquier , Ayesha , Blue Bird , Forever Pink , Fuji Waterfall ( Fujinotaki ), Miyama-yae-Murasa-ki , Seafoam , Taube , Tricolor and Veitchii ] wererated resistant (R) or moderately resistant to leafspot under each of the three environments. In 2007 2008, approximately 51% of the cultivars were ratedR in full shade, but only 5% were R in full sun. In2009 2010, only 1% of the cultivars were rated R inpartial shade. Although environmental parametersincluding temperature and rainfall influence diseaseseverity and host reaction, a shaded environmentwas least favourable for leaf -spot disease develop-ment, which demonstrates that establishing hydran-gea in shaded environment can be an effective toolalong with cultivar selection for managing leaf -spotdiseases on Hydrangea .

3 Six pathogens,Corynesporacassiicola,Cercospo raspp.,Myrothecium roridum,Glomerella cingulata(Anamorph:Colletotrichum glo-eosporioides),Phoma exiguaandBotrytis cinerea,were associated with leaf - spot diseases of gardenhydrangea. Of the leaf -spot pathogens,C. cassiicolawas most frequently isolated (55% of all isolates),followed byCercosporaspp. (20%) and otherpathogens (25%). Because symptoms attributed toeach leaf -spot pathogen were similar, cultivarswere selected for Resistance to multiple Hydrangea ( Hydrangea macrophylla ), whichincludes bigleaf Hydrangea , French Hydrangea andflorist Hydrangea , is a popular landscape shrubplanted in full sun, partial shade or shade, and it isalso grown as a potted plant. Flowers of some culti-vars can be dried and used in floral arrangementsand crafts. It is commonly believed that hydrangeasneed only moisture and shade to thrive, but most cul-tivars benefit from dappled direct sunlight or expo-sure to morning sun (Vintage Gardens 2008).

4 Hydrangeas are easy to grow and provide colour ingardens from mid-summer through autumn, and theycan be used as specimen plants in shrub 700 cultivars ofH. macrophyllaare described,but only about one-fourth are available in the USA(Dirr 2004; van Gelderen and van Gelderen 2004).Hydrangeas grow from to m in height with asimilar spread and produce large flowers in early tomid-summer. While four subspecies ofH. macrophyllahave been recognized (McClintock 1957), only twosubspecies, serrata, are grown commerciallyin the USA. The two subspecies can be differentiatedby their leaf and inflorescence sizes (McClintock1957), a combination of several qualitative and quan-titative morphological traits (Bertrand 2000), ormolecular markers (Reed and Rinehart 2007). Hydrangea native toJapan and is found in coastal areas from sea levelto approximately 150 m in elevation (Bailey 1989). Hydrangea , also known asmountain Hydrangea , is found at higher elevations upto 1500 m and is cultivated primarily as a gardenplant.

5 Although several inconsistencies are associatedwith the cultivar names (Haworth-Booth 1984), a cul-tivar checklist is available in Bertrand (2001).J Phytopathol160:88 97 (2012)doi: 2011 Blackwell Verlag GmbHLeaf- spot diseases can have significant impact onhydrangea health, appearance and market value. Inthe landscape, leaf - spot diseases cause unsightlyappearance of foliage. Severe defoliations are sporadicand have been associated with frequent defoliations can reduce overall plant vigour(Vann 2010). In nursery production of Hydrangea , leaf - spot diseases become problematic when plants areexposed to frequent rainfall or overhead irrigation(Williams-Woodward and Daughtrey 2001). Variousfungal pathogens cause leaf - spot diseases inH. macro-phylla(Sinclair et al. 1987; Hagan and Mullen 2001;Williams-Woodward and Daughtrey 2001). Fungalpathogens associated with Hydrangea leaf spots includeAscochyta,Botrytis,Cercospora,Col letotrichum,Cory-nespora,Phyllostictaand Septoriaspecies (Williams-Woodward and Daughtrey 2001).

6 Cercospora leaf spotcauses heavy damage on container-grown plants innursery production, and once the disease becomesestablished in an area, disease outbreaks are likely tooccur every year (Hagan and Mullen 2001). Hotweather and frequent rain showers favour disease out-breaks (Hagan and Mullen 2001; Hagan et al. 2004).Thus, the environment of south-eastern USA is highlyfavourable to Hydrangea leaf - spot diseases , and themain limiting factor to leaf - spot diseases is the avail-ability of inocula or susceptible hosts. The objectivesof this study were to (i) evaluate for Resistance to leaf -spot dis-eases and (ii) identify pathogens involved with leaf - spot diseases and MethodsPlant materialsThis study was conducted at Tennessee State Univer-sity, Otis L. Floyd Nursery Research Center (TSU-NRC) in McMinnville, TN, USA. Two sets of rootedcuttings of 88 commercial cultivars ofH. macrophyllawere grown in , plastic containers. The study com-prised 68H. (mac), 17H.

7 (serrata) cultivars and threehybrids between the two subspecies (Table 1). Sub-species assignment for cultivars used was adoptedfrom nursery preferred names (Mallet et al. 1992; Mallet1994), cultivar checklist (Bertrand 2001) and resultsfrom molecular studies by Reed and Rinehart (2007).The first set of plants was maintained in a hoop housethat was covered on all sides with 65% shade cloth(2007 2008). The second set of plants was maintained ina shadehouse open to full sun for 2 years (2007 2008)and then covered with 65% shade cloth and protectedfrom afternoon sun by two rows of taller lilac plants(2009 and 2010). At the end of each growing season,plants in the open shadehouse (exposed to full sun or inpartial shade) were moved to an enclosed shadehousefor winter protection. In early May, after the frost-freedate, plants were moved back to the open shadehouseand exposed to full sun or in partial shade as plants in this study were arranged in a complete,randomized block design, with a replication of threeplants per cultivar in the hoop house (shaded environ-ment) and five plants per cultivar in the open shade-house (full sun and partial shade environments).

8 Whilethe arrangement of plants in the hoop house was con-stant with neighbouring plants throughout the study,randomization of plants in the open shadehouse chan-ged every year as plants were moved out of the over-wintering house. All plants were fertilized in early Mayusing a 19-5-9 Osmocote Pro fertilizer (Scott s-SierraHorticultural Products Co., Maryville, OH, USA), at arate of 143 g per pot. All plants were watered usingspray stakes; plants in full sun and partial shadereceived additional 7-min, overhead sprinkler irrigationduring mid-afternoon to enhance leaf -spot diseasedevelopment. All plants were cut back in earlyFebruary before bud development and evaluationDevelopment of leaf -spot symptoms on different culti-vars was monitored. disease development was attrib-uted to natural inocula from airborne spores. Diseasesymptoms were characterized, and pathogens associ-ated with different symptom patterns were isolated andidentified. disease severity was rated monthly begin-ning in July and ending during late September or earlyOctober.

9 Although different symptom patterns wereobserved, it was impossible to separate the severity ofdifferent symptoms; however, an overall leaf -spot dis-ease severity was evaluated. disease -severity ratingused a scale of 0 5, in which 0 = no infection,1 = 1 10, 2 = 11 25, 3 = 26 50, 4 = 51 75 and5 = 76 100% of leaf area showing symptoms. Thehighest numerical rating observed during each growingseason was used as a measure of cultivar susceptibilityfor that season. Mean disease severity from 2 years data was used as a measure of cultivar susceptibilityunder each environment (Table S1). Cultivars werecategorized as resistant (R) when mean disease -severityrating was between and , moderately resistant(MR) when disease severity was between and ,moderately susceptible (MS) when disease severity wasbetween and and susceptible (S) when diseaseseverity was between and of leaf -spot leaf -blight pathogensIsolation of pathogens from surface-disinfested leaveswas performed by excising small tissue pieces from themargin of necrotic lesions and following standard iso-lation techniques (Dhingra and Sinclair 1995).

10 In addi-tion, symptomatic leaves and some stem pieces wereincubated in moist chamber for 48 72 h at room tem-perature (21 23 C), after which they were observedunder a dissecting microscope for the presence of coni-diospores and or fungal fruiting structures. A sterileneedle was used to aseptically transfer representativeconidiospores onto water agar and potato dextroseagar (PDA) where they were allowed to grow beforeisolates were subcultured into pure fungal isolates were derived from differentdisease symptom types and grown on PDA. Individual89 Evaluation ofHydrangea macrophyllafor disease ResistanceTable 1 Evaluation for cultivar susceptibility to leaf - spot diseases in garden Hydrangea ( Hydrangea macrophylla ) at three environmentsCultivaraSubspeciesbFull shadeFull sunPartial shadeDiseaseseveritycDiseasereactiondDis easeseveritycDiseasereactiondDiseaseseve ritycDiseasereactiond Akishino Temari All Summer Beauty Altona Amagi Amacha Amethyst Ami Pasquier Ayesha Bailmer ( EndlessSummer ) Beaute Vendo moise Benigaku Blauer Prinz ( BluePrince ) Blaumeise Blue Billow Blue Bird Blue Deckle Blue Wave Bodensee Bouquet Rose Charme Coerulea Diadem Domotoi Dooley Enziandom Fasan Forever Pink Frillibet Fuji Waterfall ( Fujinotaki )