CHAPTER 16

Pear Decline Phytoplasma

E. Seemüller, B. Schneider, and B. Jarausch

IntroduetionPear decline (PD) is one of the most important diseases of

pear. It is induced by a phytoplasma and was first describedin some detail in British Columbia (McLarty, 1948). Only afew years later, PD was observed in central Washington andsubsequently spread further south to Oregon and was foundin California in 1957 (Nichols et al., 1960; Woodbridge et al.,1957). There is a clear link between the occurrence of the dis­ease and the spread of pear psylla (Cacopsylla pyricola) alongthe Pacific coast. The vector was introduced from Europe andhas been present in eastern North America since the 19th cen­tury. It was first discovered in Washington State in 1939. Theoccurrence of the insect preceded the spread of the disease to­ward the south, reaching California by 1953. There is indica­lion that not only the vector but also the pathogen is native toEurope. A decline-like disorder called moria dei pero ("deathof the pear tree") has been observed in Italy since at least 1908(Catoni, 1947; Mader, 1908).

Because PD does not induce specific symptoms in Pyruscommunis (French pear) scion cultivars, the cause of the dis­ease remained obscure for many years. Several abiotic factors,fungal root rot, root nematodes, and a pear psylla toxin wereconsidered to be involved (for review see Seemüller, 1992).Transmission by grafting or by the pear psylla resulted in theconclusion that the disease is induced by a virus (Jensen et al.,1964) until phytoplasmas, then referred to as mycoplasmalikeorganisms (MLOs), were identified in diseased trees (Hibinoand Schneider, 1970). Restriction fragment length polymor­phisms (RFLP) and sequence analysis of ribosomal DNA(rD A) revealed that the disease is induced by a distinct phyto­plasma that is closely related to the apple proliferation (AP) andto the European stone fruit yellows (ESFY) agents, the causesof two other major diseases of temperate fruit trees (Seemülleret al., 1998c). Unlike most or all other phytoplasmas, whichare transmitted by either leafhoppers (Cicadellidae) or plan­thoppers (Fulgoridea), these three fruit tree phytoplasmas aremainly or only spread by psyllids (Psyllidae) (Carraro et al.,1998; Frisinghelli et al., 2000; Jensen et al., 1964; Tedeschi etal., 2002). Recently, the PD phytoplasma was taxonomically de­lineated under the provisional status 'Candidatus' for uncultur­able bacteria as 'Candidatus Phytoplasma pyri' (Seemüller andSchneider, 2004). A distinct but closely related phytoplasmahas recently been identified in decline-affected Japanese pear(P. pyricola) in Taiwan (Liu et al., 2007). Moreover, a decline­like disorder of French pear associated with a sweet potatolittle leaf-related phytoplasma has been reported in Australia(Schneider and Gibb, 1997).

77

Taxonomie PositionIn the current taxonomy 'Ca. P. pyri' is assigned, together

with 'Ca. P. mali' (the AP agent), 'Ca. P. prunorum' (the ESFYagent), and the peach yellow leaf roll (PYLR) agent, to the appleproliferation or 16SrX group (Lee et al., 2000; Seemüller andSchneider, 2004). The newly identified PD agent from Taiwan(PDTW) is also a member of this group (Liu et al., 2007).Sequence alignment revealed that, at the 16S rDNA level, 'Ca.P. pyri' is a very homogeneous taxon. The sequences of eightstrains from Germany and Italy were identical or nearly iden­tical, showing similarity values of 99.9-100%. There is noindication of sequence heterogeneity between the two rRNAoperons of the PD agent (Seemüller and Schneider, 2004).

Phylogenetically, 'Ca. P. pyri' is closely related to the otherAP group members. In interspecific comparisons of the PDIPYLR, PD/PDTW, PD/AP, PD/ESFY AP/PDTW, and ESFYIPDTW agents, differences in 16S rDNA sequences were 0.4,0.9-1.1, 1.0-1.1, 1.2-1.3, 1.1-1.3, and 1.2-1.4%, respectively(Liu et al., 2007; Seemüller and Schneider, 2004). This data in­dicates that 'Ca. P. pyri' and PDTW differ from each other like'Ca. P. pyri' from the other AP-group species. Thus, PDTWcan be regarded as a distinct taxonomic entity. Three other phy­toplasmas that cluster in the same subclade as the AP groupmembers are more distantly related: 'Ca. P. spartii' (associatedwith spartium witches' broom), 'Ca. P. rhamni' (associatedwith buckthorn witches' broom), and 'Ca. P. allocasuarinae'(associated with allocasuarina yellows). They share between94 and 97.2% 16S rDNA sequence similarity with 'Ca. P. pyri'(Marcone et al., 2004). The spacer region between the 16Sand 23S rRNA gene, another ribosomal marker, is in the up­stream and downstream region of the conserved tRNA IIe geneslightly more variable than the 16S rRNA gene. The sequenceidentity values between 'Ca. P. pyri' and the other fruit treephytoplasmas range from 95.2 to 98.8%, whereas the dissimi­larities with the other phytoplasmas clustering in the AP sub­clade are greater than 10% (Marcone et al., 2004; Seemüllerand Schneider, 2004). On the basis of deduced amino acidsequences of an immunodominant membrane protein, 'Ca. P.pyri' is more cJosely related to 'Ca. P. mali' than to 'Ca. P.prunorum' (Morton et al., 2003).

Eeonomie ImpactThe impact of the disease strongly depends on the rootstock

the trees are grafted on. During the PD epiphytotic in the peargrowing areas along the Pacific coast of North America inthe 1950s and 1960s of the last century, thousands of acres of

78 Chapter 16

mature, productive pear trees on oriental rootstocks (P. pyri­cola and P. ussuriensis) were destroyed and, in some areas,pear production was reduced by half. Trees on P. communisand P. calieryana rootstocks were considerably less affected(Blodgett et al., 1962). However, more than 50,000 trees onP. communis rootstocks were killed in northern Italy duringthe epiphytotic from 1945 to 1947 (Refatti, 1964). Also on P.communis stocks, tree losses of 20-40% and a severe reduc­tion of yield and vigor in the remaining trees were recorded inGermany (Seemüller et al., 1986; Spaar et al., 1972). In Italy,trees on quince (Cydonia oblonga) rootstocks were also se­verely affected when psyllid infestation was high (Poggi Polliniet al., 2001).

SymptomsSymptom expression is significantly inftuenced by the root­

stock and the stage of the disease. Basically, three differentforms can be distinguished: quick decline, slow decline, andreddening of the foliage that is often associated with leaf cur!.None of these syndromes is sufficiently specific to allow a reli­able diagnosis in cultivars of French pear. However, specificsymptoms are expressed by indicators such as Precocious (Fig.16.1A) (Schneider, 1977), where cork layers along midribs andleaf veins are formed upon infection. The symptoms inducedby PDTW in Japanese pear are largely similar to those causedby 'Ca. P. pyri' in French pear except for some differences inleaf deformation (Liu et al., 2007).

Fig. 16.1. Pear decline symptoms. A: Vein enlargement and browning on Pyrus communis cv. Precocious(right, healthy). B: Leaf curl (Iett, healthy). C: Small fruit symptom (right, healthy). D and E: Foliar redden­ing. F: Premature leaf drop (center).

Quick decline is the sudden wilt and death ofthe trees withina few day or weeks and is often preceded by slow decline orreddening. Quick decline may occur in summer or fall; it ismore common when trees are under stress because of hot, dryweather. Sometimes, mainly when the first symptoms appearlate in the season, the trees may die over winter or the followingspring. Quick decline is especially prevalent on trees on the ori­ental rootstocks P. pyricola and P. ussuriensis, which, becauseof their sensitivity, are no longer used in new plantings. Quickdecline mayaiso occur on trees on P. communis rootstocks, butthere is a considerable variability in this species. Sudden deathi rare on Bartlett seedlings but occurs on KirchensalIer seed­lings. Also, OHxF clonal rootstocks selected from progeniesof Old Home x Farmingdale crosses differ greatly in this re­spect. Mainly on trees on oriental stocks, a distinct brown lineon the cambial side of the bark may be observed at the union ofthe scion and rootstock (Blodgett et al., 1962; Seemüller et al.,1986; Seemüller et al., 1998b).

Slow decline occurs on oriental and on the less tolerant P.communis rootstocks. It is a progressive weakening of thetrees and it varies in severity. Terminal growth is reducedor nil; leaves are few, smalI, leathery, and light green, andtheir margins are slightly rolled up. Fall coloration may bered or the normal yellow. At the beginning of the disease, thetrees may bloom heavily, but the size of the crop is often re­dueed. As the disease progresses, both fruit set and size (Fig.16.lC) become poor. Trees may live for many years or maydie within a few years after disease appearance (Woodbridgeet al., 1957).

Reddening ofthe foliage (Fig. 16.1D and E) in late summerorfall is a mild form of slow decline and occurs on trees on moretolerant stocks such as P. betulifoliae, P. calleryana, and lesssusceptible clones and seedlings of P. communis (Griggs et al.,1968). Affected trees may be reduced in vigor, yield, and fmitsize (Fig. 16.lC). Often, the reddening syndrome is associatedwith leaf curl, a symptom characterized by downward curlingof the leaves, with margins rolled upward along the longitudi­nal axis (Fig. 16.1B). The leaf veins may be thickened, and theleaves are often crinkled and usually drop sooner than leavesofhealthy trees (Fig. 16.IF). The symptoms may ftuctuate con­siderably over the years, and symptom remission may occur.The slow decline syndrome mayaiso develop. Reddening isuncommon for healthy trees on highly compatible rootstockssueh as P. communis, but it may occur on healthy trees onguinee rootstocks. However, reddening of diseased trees onguince appears several weeks earlier than that of healthy trees(Seemüller, 1990).

Under the climatic conditions of Germany, the expression ofthe reddening and leaf curl syndromes is associated with theseasonal ftuctuation of the phytoplasma population in aerialparts of infected trees. Phytoplasmas depend on functionalphloem sieve tubes, and because the sieve tubes in the sterneease to function in late autumn and early winter, the pathogenis eliminated in the aerial parts du ring winter. It survives in themots, where functional sieve tubes are present throughout theyear. From the roots, the scion may be recolonized in springwhen new phloem is being formed. However, recolonizationmay be weak or may not occur every year and may be miss­ing for several years. Because expression of foliar symptomsdepends on the presence of phytoplasmas, trees intensivelyeolonized in the aerial parts usually develop characteristicsymptoms, whereas those only partially, weakly, or not colo­nized develop mild or no symptoms (Schaper and Seemüller,1982; Seemüller et al., 1984a). In Spain, the PD phytoplasmawas detected in aerial parts by nested PCR throughout the year.The pathogen was transmitted by chip budding in January andFebruary (Errea et al., 2002; Garcia-Chapa et al., 2003). Thus,

Pear Decline Phytoplasma 79

it is possible that overwintering of 'Ca. P. pyri' in the stern isinftuenced by c1imatic conditions.

Depending on the severity of disease, the root system mayalso be affected. On trees with quick decline symptoms, thefeeder roots die and so do most smaller roots up to 4 mm indiameter. Also, some of the larger roots die. Slow decline­diseased trees have a reduced number of feeder roots and,mainly after several years of disease, a much smaller root sys­tem than healthy trees. Also, the root system of trees showingthe reddening syndrome over long periods of time may be re­duced in size (Batjer and Schneider, 1960; Nichols et al., 1960).

Host Range'Ca. P. pyri' has been identified in naturally infected root­

stocks and scion cultivars of P. communis and P. pyricola andin rootstocks or own-rooted trees of P. ussuriensis, P. callery­ana, P. elaeagrifolia, and quince (Batjer and Schneider, 1960;Blodgett et al., 1962; Kunze and Seemüller, 1971; Schneider,1970; Schneider, 1977). By graft inoculation with P. commu­nis scions, the PD agent was transmitted to progenies of thefollowing Pyrus species: P. amygdaliformis, P. balansae, P.betulifolia, P. boisseriana, P. bretschneideri, P. calleryanavar. tomentella, P. calleryana var. gracilifolia, P. caucasica,P. cordata, P. cossonii, P. cuneata, P. fauriei, P. gharbiana,P. hondoensis, P. kunoriana, P. x lecontei, P. longipes, P.x michauxii, P. nivalis, P. pashia, P. pyraster, P. serrulata,and P. syriaca. Thus, it appears that most or all Pyrus speciesare hosts of 'Ca. P. pyri' (Seemüller et al. , 1998b; Seemülleret al., 2008). By Cacopsylla feeding, 'Ca. P. pyri' was trans­mitted to periwinkle (Catharanthus roseus) (Kaloostian et al.,1971) and via Cuscuta odorata bridges to periwinkle and to­bacco (Nicotiana occidentalis and N. tabacum) (Marcone etal., 1999). Using several suitable restriction enzymes, a phy­toplasma showing the same 16S rDNA RFLP profiles as 'Ca.P. pyri' was identified in European hazel (Corylus avellana)and hawthorn (Crataegus monogyna) (Marcone et al., 1996;E. Seemüller, unpublished results).

TransmissionPD is efficiently transmitted by grafting tissue from a suit­

able host such as P. communis to a recipient tree. Usually, bet­ter results are obtained by scion grafting than by budding orchipping. Root tissue is often better inoculum than stern tissuebecause phytoplasma titer in the roots is often higher than inthe stern, especially when the donor tree is nonsymptomatic.Also, unlike sterns, roots are not subjected to seasonal ftuctua­tion of the phytoplasma colonization. Before inoculation, thepresence of the pathogen in the scionwood should be verifiedusing a microscopic method or PCR. Grafting of tissue from alow-titer host such as quince usually results in low transmissionrates (Poggi Pollini et al., 1995; Seemüller et al., 1986).

Like the other fruit tree phytoplasmas of the AP group, 'Ca.P. pyri' is transmitted in nature by Cacopsylla species, wh ichalso belong to the most serious pests of pear. Jensen et al. (1964)were the first to report that C. pyricola (Förster) was vector­ing the PD agent in the pear-growing areas of the Pacific coastof North America. Other pear-feeding psyllids are not knownin the United States and Canada. In Europe, pear is the majorhost of three psyllids: C. pyricola, C. pyri (L.), and C. pyrisuga(Förster). Of these, the vectorship of C. pyricola has been con­firmed for Europe (Davies et al., 1992). In addition, C. pyri wasidentified as a major 'Ca. P. pyri' vector in France (Lemoine,1991), Italy (Carraro et al., 1998), and Spain (Garcia-Chapa etal., 2005). It is unknown whether C. pyrisuga is also involvedin the spread of the PD agent.

80 I Chapter 16

C. pyricola and C. pyri have a palaearctic distribution. Bothoccur from western Europe through Japan and China. C. pyri­cola was introduced to North America in the early 19th cen­tury and later to Argentina. In many pear-growing regions ofEurope, C. pyricola and C. pyri occur in parallel. However, C.pyricola is prevalent in the more northern regions whereas C.pyri is often predominant in central and southern parts. Bothspecies are oligophagous on Pyrus species such as P. commu­nis, P. eleagrifoLia, P. pyraster, P. amygdaliformis, and P. sal­icifolia (Burckhardt, 1994; Lauterer, 1999).

Also, the biology of C. pyricola and C. pyri is similar. Bothspecies overwinter as adults on trunks and branches of treesin pear orchards or on pear and other trees in their vicinity. Insouthern and eastern Europe, C. pyri is also reported to overwin­ter in the egg stage (Vondnicek, 1957). Infested psyllids retainthe pathogen over winter so that they are infectious when they re­start feeding on pear (Blomquist and Kirkpatrick, 2002; Carraroet al., 2001). Eggs are laid on pear shoots and spur from latewinter to early spring, and hatching starts at about the green-tipstage and goes on to about petal fall. After five nymphal instars,the adult stage is reached several weeks later. Both species arepolyvoltine. P. pyri has three to five and P. pyricola has two tofive generations annually. Eggs of the summer brood are nor­mally deposited along the mid-vein on the upper leaf surface.Adults are very active but do not move far from horne trees un­less conditions such as overcrowding or nutritional depletion ofthe foliage trigger a mass exodus from the tree or the orchard(Burckhardt, 1994; Conci, 1993; Lauterer, 1999).

Studies to analyze infection rate and transmission parame­ters have been carried out for both C. pyricola and C. pyri. Inearly transmission experiments with C. pyricola, noninfestedadults were caged on diseased trees for 1-2 months to acquirethe pathogen. Then, adults were transferred to healthy treeswhere they fed on for 5-8 days. First symptoms were observedafter 56 days. By the end of the growing season, 83% of thetest trees were symptomatic. It was also shown that the psyl­lids acquire the pathogen in a few hours of feeding and remaininfective for probably the whole life of the vector (Bailay etal., 1965; Jensen et al., 1964; Jensen and Erwin, 1963). Usinga PCR assay, 'Ca. P. pyri' was detected in both the winter formand the summer generations of C. pyricola, with no clear sea­sonal differences. The number of phytoplasmas per psyllid wasestimated to range from 1 x 106 to 8.2 X 107 cells, with highertiters in the winter form. It was coneluded that psyllid-mediatedspring infections could happen weil before 'Ca. P. pyri' wouldnormally recolonize the upper part of the tree from the roots(Blomquist and Kirkpatrick, 2002). In England, an average of17% of C. pyricola were found to be infested with 'Ca. P. pyri.'The highest infestation rates were in late spring and fall (Daviesand Eyre, 1996). In Italy, 'Ca. P. pyri' was detected in 55%of the groups collected from March through October. Acrossthe entire growing season, 30% of the test plants inoculated byexperimental psyllid feeding became infected. The pathogenwas not transmitted during winter dormancy (Carraro et al.,2001). According to Garcia-Chapa et al. (2005), the percentageof infected C. pyri is similar from June to August but reaches arate of nearly 100% in September. This data coincides with thephytoplasma titer in the aerial plant parts.

Vector transmission of the PDTW agent still awaits elucida­tion. However, the causative agent was identified by PCR andsequence analysis of PCR products from C. qianLi and C. chi­nensis collected in Japanese pear orchards. C. qianLi occurredin low numbers but contained high phytoplasma titers whereasC. chinensis was very abundant but showed low titers. Thus, C.qianli is more likely to be a PDTW phytoplasma vector (Liu etal.,2007).

Geographical DistribuitionIn Europe and North America, 'Ca. P. pyri' probably oc­

curs wherever pear is grown. The disease is also present inthe Anatolian part of Turkey (Gazel et a1., 2007) and prob­ably in other non-European pear growing regions around theMediterranean Sea. There is an unconfirmed report fromArgentina of its presence (Sarasola, 1960). The PDTW agenthas only been recorded in Taiwan (Liu et al., 2007).

DetectionSymptoms similar to those of PD can be produced by other

factors such as incompatibility, girdling, poor drainage, mal nu­trition, winter injuries, and drought. Also, the brown line at thebud union does not occur consistently. Therefore, diagnosis hasto be confirmed by other means ineluding indexing, micros­copy, and molecular techniques. Confirrnation is possible byindexing using woody indicators such as Precocious or otherP. communis seedlings that develop specific vein symptoms(Fig. 16.1A) (Schneider, 1977). Stern scions can be used whengrafting is done in late summer or fall and the donor trees arefully symptomatic. Otherwise grafting of root scions is rec­ommended as phytoplasma titers in roots are generally higherthan in aerial parts and as roots are, unlike the stern, colonizedthroughout the year (Seemüller et al., 1984b). The plants shouldbe kept in the greenhouse at temperatures between 25 and 30°Cand observed for two growing seasons (Seemüller, 1989). Thelengthy procedure and low sensitivity are major disadvan­tages of the indexing approach. According to Waterworth andMock (1999), nested PCR replaces 3-year tests with sensitiveindicators.

Infection can also be detected by electron microscopy(Hibino and Schneider, 1970). However, expensive equipment,time requirements, and low sensitivity prevented the wide useof this technique. A quick and widely used method is the de­tection of the pathogen by ftuorescence microscopy using theDNA-binding ftuorochrome 4/-6-diamidino-2-phenylindole(DAPI) (Seemüller, 1976). Phytoplasmas are nonspecificallystained and appear as brightly ftuorescent particles in the sievetubes. It is important to evaluate only the ftuorescence in thesieve tubes since other cell types also contain ftuorescent struc­tures such as nuelei, mitochondria, and plastids that are absentin mature sieve tubes. Addition of aniiine blue as a second stainfacilitates recognition of sieve tubes. The sensitivity of theDAPI methods is mostly satisfying. However, very low phyto­plasma titers such as, for instance, those occurring in guince,are difficult to detect. No immunological detection procedureis available for 'Ca. P. pyri.'

D A-based techniques became available when protocolsfor the enrichment of phytoplasmal DNA and their subsequentcloning were implemented (Kirkpatrick et al., 1987; Kollaret al., 1990). Currently, the PCR technology is the method ofchoice in terms of sampie preparation, throughput, specificity,and sensitivity. For successful detection, template DNA is ex­tracted from petioles and midribs or from phloem prepared fromstern or root tissue as described (Ahrens and Seemüller, 1994).From aerial parts, the pathogen can be detected most reliablyin late summer and fall or in roots throughout the year. DNAextraction is often performed using a cetyltrimethylammoniun­(CTAB-) based protocol (Ahrens and Seemüller, 1992; Doyleand Doyle, 1990). Commercial kits are also employed.

For sensitive PCR ampli fication, universal phytoplasmaprimers and specific primers both derived from the 16S rDNAsequence and the intergenic 16S-23S rDNA region are mostwidely used. However, due to the e10se relationship of 'Ca.P. pyri' with 'Ca. P. mali' and 'Ca. P. prunorum,' most of the

specific primers show cross reactivity with the DNA of otherAP-group fruit tree phytoplasmas, especially with 'Ca. P. mali.'Differentiation of the pathogens can be achieved by RFLPanalysis with freguently cutting restriction enzymes such asSspl, Rsal, Sfel, and BsaAI (Firrao et al., 1994; Lee et al., 1995;Lorenz et al., 1995; Seemüller et al., 1998a; Smart et al., 1996).One of the ribosomal primer pairs (fPD/rPDS) shows a higherspecificity and does not cross-amplify DNA from 'Ca. P. mali.'However, not all PD phytoplasma strains can be detected withthese primers (Lorenz et al., 1995). The c10sely related 'Ca. P.pyri' and the PYLR phytoplasma cannot be distinguished withany primer combinations at present. A seguence analysis of the16S rRNA gene is the only means to distinguish the two patho­gens. Primers based on non-ribosomal seguences are availableas weil. However, they are either not PD-specific or do not initi­ate amplification of all strains (Jarausch et al., 1994; Lorenzet al., 1995). Nested PCR using a succession of amplificationswith either two universal primer pairs or one amplification witha set of universal primers and another one with a pair of spe­eifie primers is often employed (Davies et al., 1996; Errea et al.,2002; Garcia-Chapa et al., 2003; Lee et al., 1995). However,the high sensitivity provided by nested PCR is normally notrequired.

ControlThe use of certified planting material is recommended to

delay the introduction of the disease. uclear stocks should besereened at regular intervals by examining root sampies usinga sensitive and specific PCR procedure. If the health status ofstock plants is doubtful, there is, according to results obtainedin Germany, virtually no risk of perpetuating 'Ca. P. pyri'when hardwood cuttings and scionwood are cut in late winterand early spring (Seemüller et al., 1984b). In summer, phyto­plasmas can be eliminated from budwood by hot water treat­ment at 47.5°C for 0.5 hour or 45.0°C for 1 hour (Adams andDavies, 1992). In young plantings and nurseries, diseased treesshould be rogued because recovery of young trees is usuallypoor. Good growing conditions, especially an adeguate supplyof water and nitrogen, improve the performance of decline­infected trees. The application of tetracycline antibiotics wassuccessfully used in the United States for PD control over manyyears. However, this laborious treatment has been discontin­ued because rootstocks less susceptible than oriental stocks areavailable now and the psyllid problem can be better managed.

The most important measure against PD is the control of thepsyllid vectors. Psyllids are not only responsible for primarytransmission of 'Ca. P. pyri,' they also influence the severity ofthe disease. The more psyllids are feeding on a tree, the greaterthe showing of decline. Little decline is observed in well-cared­for mature orchards where the psyllids are excluded, even if ahigh percentage of the trees are infected. If psyllid infestationis reduced from heavy to light, affected trees on resistant stockswill start to regain vigor. It appears that infested pear suckersreinfect the trees and initiate or enhance colonization of the topin spring. Thus, recolonization of the top starts earlier or willbe more intensive than when it occurs via the roots.

The goal of the psyllid management program is, therefore, tohold the vector population to a level as low as possible through­out the season. There are several strategies to reach this goal,which depend on factors such as climate, level of population,psyllid migration, significance of predators in the manage­ment program, and the occurrence of pesticide resistance. Thepreferred strategy is based mainly on a treatment against theoverwintering adults that may reinfect the trees early in theseason. This spray has to be applied before egg-Iaying starts.

Pear Decline Phytoplasma 81

When psyllid counts remain high, further sprays are needed,mainly until Mayor June. Selective chemicals that are leastdamaging to beneficial insects should be used. If a high sur­vival of predators is ensured, no further sprays are needed laterin the season. Another strategy in psyJlid management concen­trates on controlling the first generation. The sprays have to bedirected against the first three nymphal stages that are mosteasy to kill. A further approach, which may be appropriate forcooler c1imates where psyllids are a less important pest, is todefer adecision on spraying until summer, when the need for aspray may be assessed more confidently by monitoring psyllidand predator abundance. However, any spray in summer hasto be selective to minimize poisoning predators which includeAnthocoris nemoralis, Coccinelidae spp., and Chrysopidaespp. (Campbell, 1986; Solomon et al., 1989; Sterck andHighwood, 1992). Breeding programs to obtain resistance toC. pyri and C. pyricola have been ongoing in Europe and NorthAmerica since 1920. Resistance can be transferred from P. us­suriensis and P. pyricola to P. communis cultivars (Pasgualiniet al., 2006).

Data on the fluctuation of PD phytoplasma colonization inthe stern and the persistence of the pathogen in the roots (seeabove) indicate that growing trees on resistant rootstocks canprevent the disease. Differences in rootstock resistance havebeen observed from the beginning of the epiphytotic alongthe Pacific coast of North America (see above). Since then,extensive screening of P. communis genotypes and progeniesof other Pyrus species and of guince genotypes has been per­formed in North America. The results obtained are summa­rized below by ranking of the various rootstocks in terms ofsusceptibility (Blodgett et al., 1962; Westwood and Lombard,1982). This ranking includes OHxF clones that are selectedgenotypes from the cross of P. communis cultivars OId Home xFarmindale (Reimer, 1950).

Highly resistant: Old Home clonal, OHxF clones, Quince Aand C, P. betulifolia seedling.

Resistant: Own-rooted Anjou, Bartlett, and Winter Nelis;seedlings of Bartlett, KirchensalIer, P. calleryana, P. elaeagri­folia, P. nivalis, P. pashia, and P. syriaca.

Susceptible: French P. communis seedlings and seedlingsof P. amygdaliformis, P. caucasica, P. cordata, and P. fauriei.

Very susceptible: Own-rooted plants and seedlings of P.pyricola and P. ussuriensis.

Some of these results were confirmed by assessing rootstockresistance in Europe. In work by Giunchedi et al. (1995), own­rooted trees of cvs. Conference, Bartlett, and Abate Fetel werelittle affected. However, own-rooted Comice trees developedsevere symptoms under the same conditions. The authors alsoreported that Comice was the most susceptible cultivar that wastested. It performed better on guince rootstocks than on P. com­munis seedlings. In their study, trees on Quince A and QuinceC suffered more from disease than trees on Quince BA29 andQuince CTS212. Previous studies by Seemüller et al. (1986)indicated that the resistance of guince is based on its poor hostsuitability for 'Ca. P. pyri,' resulting in low phytoplasma titersor the elimination of the pathogen from the rootstock. The poorsurvival in guince was confirmed (Poggi Pollini et al., 1995).There is evidence that this low titer impairs recolonization ofthe stem in spring, resulting in no or a milder form of disease.For this reason, guince was assessed as fairly resistant root­stock in Germany (Seemüller et al., 1986). However, trees onguince are often severely affected in ltaly. These differencesmay be explained by a heavier psyllid problem under the Italianconditions. It is supposed that under these circumstances thetrees are repeatedly reinfected from the beginning of the grow­ing season. Therefore, the top of the tree is infected earlier and

82 Chapter 16

more heavily than when recolonization takes place from theroots. This results in a higher severity of disease (Poggi Polliniet al., 2001). In Germany, psyllid infestation is less and so is theseverity of disease of trees on quince.

Like in North America, considerable variability ofresistancewithin P. communis was identified in Germany (Seemüller etal., 1998b). Examining four seedling populations, one of themproved highly resistant, KirchensalIer seedling and anotherprogeny were moderately resistant, and a progeny of French cv.Feudiere was susceptible. There were also great differences inthe resistance of some OHxF rootstocks, which all are reportedto be highly resistant by Westwood and Lombard (1982).Following experimental inoculation and an observation periodof eight years, only OHxF 87 showed satisfactory resistance.OHxF 333, 18, and 267 were moderately affected while OHxF217 and 69 proved very susceptible, showing mortality ratesof 44% and 64%, respectively, in the year following inocula­tion. Fruit size was little affected on trees on OHxF 18, 69,and 87 rootstocks. Fruit size was moderately reduced on OHxF267 and severely reduced on OHxF 217 and OHxF 333 stocks(Seemüller et al., 1998b). The poor performance of trees onOHxF 217 and 333 was also observed elsewhere (Azarenko etal., 2002; Giunchedi et al., 1995).

Besides P. communis, several other Pyrus species includ­ing P. betulifolia, P. calleryana, P. cordata, P. pyricola, andP. ussuriensis have been used as rootstocks. To examine theresistance of possible rootstocks other than those based on P.communis, progenies of 36 open pollinated genotypes of 26Pyrus species and subspecies listed above under "Host range"were examined under experimental inoculation conditions(Seemüller et al., 1998b, 2008). The material tested includedspecies reported to be resistant, such as P. betulifolia, and spe­cies known as highly susceptible, such as P. pyrifolia and P.ussuriensis. Following graft inoculation and observation over18 years, considerable differences in pear decline resistance be­tween and within the progenies were observed. Unaffected orlittle-affected and moderately to severely affected trees were ob­served in all progenies. However, great quantitative differencesamong them were observed. In the progenies of about one thirdof the seed parents, most of the individuals showed a high levelof resistance to grafted trees. Significantly different from thisgroup was another third of the progenies that mostly showedhigh susceptibility in grafted trees. Between these two groupsthere were progenies that statistically neither differed from theresistant nor from the susceptible group. These progenies weredefined as moderately resistant. Significant differences in re­sistance were also observed between progenies of genotypesof the same species that originated from different locations. Asdescribed above for P. communis, P. betulifolia sources fromIbaraki, Japan, Corvallis, Oregon, and Dortmund, Germany,were classified as being highly resistant, moderately resistant,and susceptible, respectively. Similarly, two progenies of twosources of P. calleryana type Bradford and P. ussuriensis wererated as resistant and susceptible, respectively. This variationindicates that resistance is a segregating trait. Therefore, seed­ling progenies of Pyrus species are unsuitable for being usedas rootstocks. Rather, resistant genotypes have to be carefullyselected, then propagated vegetatively.

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