These values are higher than for the majority of domesticated fruits commonly reported, closest to very sweet fruits like pineapple and papaya ,access date 19 April 2010. The mean value for °Brix increased 22% from wild to cultivated fruits, from 13.6 to 16.6. It is interesting to note that even the wild fruits are very sweet, which may have predisposed this species to early selection as a cultivar. The reduction we saw in levels of phenolics in the pulp is consistent with a human preference for fruits with less bitterness . However, while a reduction in phenolics may indicate direct selection for palatability, it may also reflect an indirect effect of selection on the production of larger, sweeter fruits. Rosenthal and Dirzo suggest that selection for yield may result indirectly in the reallocation of energy away from defense, because of physiological constraints and tradeoffs. While all these differences were significant, the Discriminant Analysis showed that the great majority of the variance distinguishing wild from cultivated classes was driven by fruit size, with sugar concentration also contributing. This suggests that humans may have selected primarily for increased fruit size in this species, a hypothesis that could be tested with ethnobotanical studies. ENVIRONMENTAL VS. GENETIC CONTRIBUTIONS Many of the traits we measured, black plastic plant pots such as fruit size or sugar content, could be influenced by environmental conditions. Cultivated trees are likely to experience less competition for light, water, and nutrients than do wild trees.
However, several lines of evidence suggest that there is a large genetic component to the traits that distinguish wild from cultivated trees. First, both classes of trees, but especially the wild individuals, came from a wide range of environmental conditions. Some of the wild trees are now in open areas because the forest was cut around them, or are on the edges of forest fragments and therefore have access to more light and possibly nutrients than wild trees in the center of the forest. When we categorize the wild trees for their location, we see no trend toward differences in °Brix between trees on edges or in the open vs. trees in closed-canopy forests . We do see evidence suggesting that edge fruits areslightly bigger, a difference of about 10 g; this is substantially lower than the 73 g difference between the wild and cultivated means. Second, despite year-to-year variation in environmental conditions such as timing and intensity of the dry season, we saw very high correlations between years in trait values, especially for traits related to fruit size. We have recently planted a broad sample of wild and cultivated genotypes in two field plantations in Panama, which in 10 to 20 years should provide a better estimate of the genetic contribution to variation in this species.For most traits, we found no difference in trait variance between wild and cultivated samples. However, we did find significantly greater variance among cultivated trees than among wild trees for fruit mass and seed number . In contrast, there was lower variance among cultivated trees for the concentration of phenolics in pulp.
Through the process of domestication, we expect to see genetic variation strongly reduced for “domestication genes,” those traits that are critical for bringing the species into cultivation, while genetic variation should increase for “crop diversification genes,” reflecting a range of preferences during selection . Domesticated plants often show more intraspecific variation than their wild relatives for traits of interest to people , even in cases where there is an overall reduction in genetic diversity due to a domestication bottleneck . The reduced variance we found for phenolics may indicate consistent selection against bitterness, while the increased variance for fruit size could possibly reflect a range of preferences by the distinct cultural groups that utilize this species in central Panama, including Emberá and Wounaan communities, as well as people of Hispanic and Antillean descent. As mentioned earlier, caimito in this region may also include some genotypes transported from the Antilles. Ethnobotanical studies aimed at understanding cultural preferences, selection targets, and selection intensity could help elucidate this issue. However, it seems more likely that the broad range in fruit size is generated by a lack of fixation of alleles associated with domestication due to a contemporary or historical influx of wild-type genes, as well as recombination among cultivated genotypes. Zohary and Spiegel-Roy have suggested that most fully domesticated tree crops are asexually propagated, at least in the Old World. This is thought to increase uniformity and reduce the production of unwanted intermediate forms, particularly as many tree crops are highly heterozygous and show an outcrossing sexual system. Chrysophyllum cainito is not propagated asexually in Panama.
The mating system of C. cainito has not been studied, but the species has small flowers that are pollinated by small bees , and we expect that it is primarily outcrossing like similar tropical lowland tree species . We suggest that the extremely wide range of variation in fruit size among cultivated phenotypes is what might be predicted for a fruit tree species in the early stages of domestication. This is consistent with the findings of Leakey et al. for the semidomesticated fruit trees Dacryodes edulis and Irvingia gabonensis. However, unlike those species, the distributions of traits in our cultivated samples of C. cainito did not show statistically significantly more skew than wild populations .We found a strong positive correlation between fruit size and both seed number and seed size. A positive correlation between fruit size and seed number has also been reported in sweet pepper and tomato , and for these species the genetic basis is known. The correlations that we observe in C. cainito may be due to pleiotropic effects, tight linkage of loci, or a combination of these factors. From a physiological perspective, fruit size may be directly related to seed number because the developing seeds are a source of auxin for the developing fruit; therefore, an increase in number of seeds results in a larger fruit . In addition, a general “gigas effect” in domesticates can produce an increase in organ size through increased cell number and/or cell size reviewed in Pickersgill 2007, which could also explain the positive correlation of fruit size and seed size/number observed in caimito. Therefore, even if human preferences were in the direction of smaller seeds, as is the case in tempesquistle , or fewer seeds, genetic and physiological factors may constrain seed size and number in cultivated C. cainito.Nearly one-third of the world’s food production is lost or wasted through the food supply chain, according to the Food and Agriculture Organization. Fungal pathogens are responsible for causing diseases like rots and molds that result in reduced product quality, shelflife, and market value, leading to significant losses of harvested fruits and vegetables in post harvest. Fungal pathogens can gain access to the fruit tissues in different ways: by establishing latent infections of flowers, exploiting wounds and natural openings , or by directly penetrating the host cuticle. Vector insects can also cause damage, which is a common entry point for pathogens. Additionally, mishandling and physical damage of the products during harvest, sorting, packing, transportation, cold storage, and retailing contribute to a higher incidence of disease. The germination of fungal spores requires moisture and stimulation from host solutes that diffuse into the initial penetration site. After germination, specific signals from the plant surface trigger the formation of specialized fungal structures that enable the pathogen to penetrate the plant cell walls and establish infection. Once the disease is well established in one fruit, black plastic planting pots it spreads quickly to adjacent healthy fruit, a process known as nesting. A well-known nesting pathogen is Botrytis cinerea, which releases airborne conidia that readily nest on damaged or senescent fruits, initiating decay and facilitating further spread. Rhizopus stolonifer, the causal agent of Rhizopus rot in various fruits and vegetables, is another prominent example of a nesting pathogen. Following spore germination, R. stolonifer produces mycelial stolons that attach to the host surface, enabling it to colonize healthy fruits and initiate infections. Penicillium spp. are commonly considered wound-dependent pathogens; however, it is commonly observed that if the fungi are initially established in a rich food source like a decaying fruit, the mycelium can readily invade the tissues of an adjacent healthy fruit. This phenomenon is how an initial low incidence of green or blue mold in a packinghouse storage facility can develop into major losses after prolonged fruit storage. Integrated pest management strategies have been developed to reduce or eliminate fruit infections, including synthetic fungicides before and after harvest, biological control agents, essential oils, cold storage, and modified atmosphere packaging.
To test the effectiveness of these strategies, reliable laboratory or field-based inoculation methods are required to obtain quantitative data that goes beyond subjective ordinal rating scales that may be influenced by human bias. While natural infections can provide insights into disease dynamics in real-world scenarios, they can be unpredictable and impacted by environmental factors, making it difficult to control and replicate experimental conditions. Therefore, the study of plant-pathogen interactions generally relies on pathogen inoculation techniques. Dip and spray are two common inoculation methods where fruits are covered in a fungal spore suspension by submersion or application with an atomizer. These methods allow for uniform, whole-fruit inoculation but may result in lower disease incidence and severity due to the challenges in standardizing the process. Another method is wound inoculation, which involves creating artificial entry points on the fruit surface before applying the fungal spore suspension or mycelial plug. This method simulates wounds and enables precise and reproducible experiments. Still, it may not accurately represent the natural infection process as it bypasses the initial steps of adhesion and penetration on the plant tissues. As of present, our comprehensive literature review has not revealed any reported methods that faithfully replicate the post harvest nesting phenomenon. This study presents a novel methodology for assessing post harvest infections of persistent fungal pathogens through contact-based inoculation of fruits. We optimized this protocol using four impactful fungal pathogens and post harvest commodities that are commonly affected by fungal disease: Botrytis cinerea and Penicillium expansum in tomato and apple, respectively, and Penicillium italicum and Penicillium digitatum in orange. B. cinerea is the causal agent of gray mold, a devastating disease that causes billion-dollar losses on fruit commodities worldwide. P. italicum and P. expansum, causal agents of blue mold, and P. digitatum, causal agent of green mold, are significant post harvest diseases. Our protocol unveils new possibilities for testing disease management strategies and studying the nesting behavior of post harvest fungal pathogens.We conducted several trials to determine the best source of fruit tissues for contact-inoculation. The developed inoculation method involves producing the source fruits through wound inoculation, and preparing the non-wounded target fruits. The selected source and target fruits ideally should not have any surface imperfections such as scars, wounds, or bruises, and apples should have an intact pedicel. Fruits were first disinfected in 10% sodium hypochlorite, rinsed twice in sterile Milli-Q, and dried with sterile tissue paper before inoculation. Source fruits were wounded multiple times on the stem end. For oranges, three equidistant wounds were created using a sterile nail following the protocol developed by Vilanova et al.. For tomatoes and apples, equidistant wounds were created in four locations with a sterile pipette tip . Oranges were inoculated with P. italicum or P. digitatum, while tomatoes and apples were inoculated with B. cinerea and P. expansum, respectively. Source fruits were incubated under high relative humidity at 10 °C for 10 and 13 days for orange, 20 °C for 4 days for tomato, and 10 days for apple. By then, the source fruits should have developed lesion sizes of about 30, 10, and 20 mm for oranges, tomatoes, and apples, respectively. In initial attempts, we used culture media plugs with well-established fungal mycelial growth as the inoculum source. However, this method was not always successful and did not accurately mimic how fungal infections occur while handling and storing fresh produce.Two different inoculum sources were explored for the contact inoculation: whole fruits and fruit tissue sections. When using whole fruits as the source, target fruits were placed on a plastic boat lying on the equatorial region.