Little is known about the histologic features of a latent Monilinia fructicola infection and brown rot in infected fruit. This report informs on the results of an investigation whose aim was to analyze the microanatomy of nectarines with a latent and visible M. fructicola infection. Mature nectarines were inoculated with an M. fructicola isolate and incubated at 25°C for 0, 24, 48, 72, or 96 hours in the dark. For investigating the latent infection process, the inoculated nectarines were first incubated at 25°C for 24 hours in the dark and then incubated at 4°C for 72, 144, 216, and 288 hours in the dark. At the end of the incubation, samples of nectarine tissue were excised from the inoculation points and prepared for light and transmission electron microscopic examinations. No signs of disease were seen on the surface of nectarines with a latent infection over the 288-hour incubation period. When the tissue samples were microscopically examined, M. fructicola colonized the stomata and this stomatal colonization progressively increased over time and was associated with gradual collapse of the epidermal cells and colonization of the subepidermis. In nectarines with visible brown rot, the disease usually appeared after 24 hours on the surface and in the uppermost layers of epidermal cells, which began to collapse after 48 hours. Subsequently, the diseased tissues of the nectarines displayed (a) colonization of the epidermis and mesocarp by M. fructicola with thin and thick hyphae, (b) collapse and disruption of epidermal and mesocarpic cells, (c) lysogenic cavities in the subepidermis and mesocarp, (d) degradation of the cuticle and epidermis, and (e) M. fructicola sporulation. M. fructicola is active during latent infections because slow and progressive colonization of nectarine subcuticular cells by the fungus occurs.

Most stone fruit with a latent brown rot infection caused by Monilinia do not develop visible signs of disease until the arrival of fruit at the markets or the consumer’s homes. The overnight freezing-incubation technique (ONFIT) is a well-established method for detecting latent brown rot infections, but it takes between 7 to 9 days. In this report, we inform on the advantages of applying a qPCR-based method to (i) detect a latent brown rot infection in the blossoms and fruit of nectarine trees (Prunus persica var. nucipersica) and (ii) distinguish between the Monilinia spp. in them. For applying this qPCR-based method, artificial latent infections were established in nectarine flowers and fruit using 10 Monilinia fructicola isolates, 8 M. fructigena isolates, and 10 M. laxa isolates. We detected greater amounts of M. fructicola DNA than M. laxa and M. fructigena DNA in latently infected flowers using qPCR. However, greater DNA amounts of M. laxa than M. fructicola were detected in the mesocarp of latently infected nectarines. We found that the qPCR-based method is more sensitive, reliable, and quicker than ONFIT for detecting a latent brown rot infection, and could be very useful in those countries where Monilinia spp. are classified as quarantine pathogens.

a b s t r a c tBrown rot caused by the fungi Monilinia laxa (Aderhold and Ruhland) Honey, M. fructicola (Winter) Honey, or M. fructigena (Aderhold and Ruhland) is a serious fungal disease of peaches. The fungal infection process begins when fungal conidia germinate on the fruit surface to produce germ tubes and/or appressoria, and the incidence of brown rot increases as fruit approaches maturity. The interaction between the fungal infection process, peach maturity, and the environmental conditions is not well understood. Accordingly, the objectives of this investigation were to investigate germ tube and appressorial formation by M. laxa and M. fructicola when they were exposed to peach skin from mature and immature fruit at various temperatures and relative humidities (RHs). The greatest number of germ tubes was found when M. laxa or M. fructicola was incubated in culture medium which contained a skin extract of mature peaches. In contrast, the greatest number of appressoria was found when M. laxa or M. fructicola was incubated in culture medium which contained a skin extract of immature peaches. Although M. fructicola produced the same number of germ tubes and appressoria at 4°C, M. fructicola produced more germ tubes than appressoria at temperatures higher than 10°C. M. laxa produced more germ tubes than appressoria at any temperature, except when it was incubated for 48 h on culture medium which contained a skin extract of immature peaches at 10°C at 80% or 100% RH, or at 25°C at 60% RH. M. laxa conidia germinated better than M. fructicola conidia at low temperatures. Germ tube and appressorial formation by Monilinia spp. were influenced by fruit postharvest handling. The number of germ tubes that were formed by M. laxa conidia was significantly greater than that for M. fructicola when the conidia were incubated at 100% RH, and this number increased after 3 days of refrigeration. The number of appressoria that were formed by both Monilinia spp. also increased after 3 consecutive days of refrigeration. Negligible or no germination of M. fructicola and M. laxa conidia occurred when the RH was 60%. We concluded that the dissimilar abilities of M. laxa and M. fructicola to germinate and form appressoria at low temperatures conferred a competitive advantage to M. laxa to survive during fruit postharvest refrigeration and cold storage at 4°C.

No abstract

Latent infections caused by Monilinia spp. in nectarines cause great economic losses since they are not detected and rejected at harvest and can appear at any time post-harvest, even at the consumer’s home. The effect of a pre-cooling chamber, water dump operation, and cold-storage chamber on the activation and/or development of preharvest latent infections caused by Monilinia spp. on nectarines were studied under different postharvest conditions: (a) cold storage for 0, 1, or 3 d at 4 °C at either 75% relative humidity (RH) or 100% RH before water dumping, (b) water dumping for 10 minutes at 15 °C, and (c) cold storage for 0, 3, or 10 d at 4 °C at either 75% RH or 100% RH after water dumping. These storage conditions were transformed to fungal physiological time. For visualization of the latent infections caused by Monilinia spp., the nectarines were placed in sterile paper bags and frozen at −20 °C for 48 h in order to damage the epidermis. To compare different handling scenarios, the incidence of latent infection was modelled for physiological time description by a modified Gompertz model. The activation and/or development of preharvest natural latent infections caused by Monilinia spp. at postharvest was mainly related to temperature and incubation time at postharvest. Storing nectarines with any postharvest handling less than 11 days at 4 °C avoids brown rot symptoms and reduced the activation and/or development of pre-harvest latent infections caused by Monilinia spp., while more cold days caused the exponential phase of latent infection activation and/or development. The Gompertz model employed could be used for predicting the activation and/or development of latent infection caused by Monilinia spp. at postharvest conditions and looks at the postharvest life. To our knowledge, this is the first time that the effects of post-harvest handling on latent infections in fruit have been studied.