Epidemiology of Monilinia laxa on nectarine and plum : infection of fruits by conidia

[摘要] ENGLISH ABSTRACT:Postharvest decay of stone fruit in the Western Cape province of South Africa iscaused primarily by Botrytis cinerea (grey mould) and Monilinia laxa (brown rot). Little isknown about the relative importance and seasonal occurrence of the two pathogens innectarine and plum orchards, the mode of penetration of fruits by M laxa, latency andsubsequent disease expression by the latter pathogen. These aspects were investigated in thisstudy.By sampling from the Unifruco Quality Evaluation Scheme and from 11 stone fruitorchards, observations were made over a 3-year period of the occurrence of grey mould andbrown rot in the major stone fruit regions. Botrytis cinerea was found to be the mostimportant pathogen causing blossom blight and postharvest decay on stone fruit. Thepathogen was most prominent on early- and mid-season culti~ars. Brown rot was exclusivelycaused by M laxa and no evidence was found that M fructicoZa had been introduced into theregion. Monilina laxa was most prominent on the later maturing cultivars. Botrytis cinereablossom infection did not contribute directly to postharvest decay. Both surface inoculumand latent infection consistently occurred on fruit in each orchard, although at fluctuatinglevels. Disease expression on developing fruit was not governed by the amount of B. cinereaoccurring on fruit surfaces, but by the ability of fruit to resist disease expression. The amountof B. cinerea on fruits was generally higher during spring than during summer. Monilinialaxa occurred sporadically on the blossoms of late-maturing cultivars. Immature fruit weregenerally pathogen-free and disease expression occurred on maturing fruit only. Thesefindings suggest that conidia of M laxa are generally produced in orchards when fruits areapproaching maturity and can penetrate and infect maturing fruit only.The behaviour of airborne M laxa conidia was subsequently studied on nectarine(cultivar Flamekist) and plum (cultivar Laetitia) fruit. For these studies, an inoculationmethod that simulates natural infection by airborne conidia was used. Fruit at pit hardening, 2 wk before harvest, harvest stage and after cold storage (nectarines 4 wk at -o.soC followedby 1 wk at 23°C at ±56% RH; plums 10 days at .....().5°C,18 days at 7.5°C followed by 1 wk at23°C at ±56% RH) were dusted with dry conidia of M laxa in a settling tower. The fruitswere incubated for periods ranging from 3 to 48 h at high relative humidity (2':93%, humidfruit) or covered with a film of water (wet fruit). Behaviour of the solitary conidia wasexamined with an epifluorescence microscope on skin segments stained in a differential staincontaining fluorescein diacetate, aniline blue and blankophor. The ability of solitary conidiato colonise the fruit surface, penetrate fruit skins and to induce disease expression wasdetermined by using a differential set of tests. For these tests, fruit were surface-sterilised(30 s in 70% ethanol) or left Unsterile. From each group, fruit were selected for isolation(skin segment test), immersed in a 3% paraquat solution (paraquat-treated fruit test) or leftuntreated (sound fruit test). 1be findings demonstrated that solitary conidia of M laxabehaved consistently on plum and nectarine fruit surfaces: appressorium formation and directpenetration was not observed on any of the fruit surfaces and germ tubes penetrated fruitpredominantly through stomata, lenticels and microfissures in the fruit skin. The monitoringof airborne conidia revealed subtle effects of the fruits on the behaviour of solitary germlings,which could not be seen when using conidial suspensions. On both fruit types, no deleteriouseffect was seen on conidial and germling survival when fruit were kept humid at pithardening, 2 wk before harvest and harvest. However, conidial and germling survival weredrastically reduced by prolonged wet incubation of fruits. The findings on disease expressionin the skin segment, paraquat-treated fruit and sound fruit tests clearly showed that the skin ofboth nectarine and plum fruits were not penetrated at the pit hardening stage, latent infectionswere not established and fruitsreacted resistant to disease expression. These facets on bothfruit types were furthermore unaffected by wetness. The barrier capacity of the fruit skin ofthe two stone fruit types however differed drastically later in the season. On nectarine, fruitskins were more readily penetrated and disease expression became more pronounced whenfruit approached maturity. Penetration and disease expression on ripening nectarine fruitwere furthermore greatly influenced by wetness. Maturing plum fruit, on the other hand, didnot display the drastic change in the barrier capacity of fruit skins as observed on nectarine.The influence of wetness on infection and disease expression was also less pronounced thanon nectarine. In fact, plum fruit remained asymptomatic in the sound fruit test afterinoculation and humid incubation at the 2 wk before harvest stage, harvest stage and aftercold storage. Plum fruit at these stages only developed disease after a prolonged period(~12 h) of wet incubation. The paraquat fruit test revealed that these fruits became moresusceptible to latent infection, but they were not as susceptible as nectarine. Collectively,these findings indicate that M. laxa fruit rot epidemics on plum and nectarine are driven byinoculum levels on fruit approaching maturity and by weather conditions prevailing duringthe preharvest and harvest period. However, the barrier capacity of plum skins isconsiderably more effective than that of nectarine fruit. Wounds would therefore play animportant role in the epidemiology of M. laxa on plum fruit.Infection of fresh wounds by airborne M. laxa conidia, and by conidia and germlingsthat have established on fruits, was therefore investigated. Plum fruit (cultivar Laetitia) at pithardening, 2 wk before harvest, harvest stage and after cold storage were dusted with dryconidia of M. laxa in a settling tower.- Infection of rionwounded fruit and of fresh wounds by\the airborne conidia on dry, humid and wet plum fruit surfaces, and by conidia and germlingsthat have been established on fruits under the wetness regimes was then investigated.Nonwounded immature and mature fruit remained mostly asymptomatic, whereasnonwounded cold stored fruit decayed readily. Wounding drastically increased infection byairborne conidia. Immature fruits were less susceptible to wound infection by the airborneconidia than mature fruits. Conidia dispersed freshly were more successful in infecting freshwounds than conidia that were deposited, or germlings that established, on fruit surfaces4 days prior to wounding. This decrease in infectivity was especially pronounced on humidand even more on wet incubated fruit. This study clearly showed that in order to reduce. theincidence of brown rot, inoculum levels on fruit approaching maturity should be reduced bysanitation practices and fungicide applications. Furthermore, it is essential to protect fruits,especially. near-mature fruits, from being wounded.

[发布机构] Stellenbosch University