A more plausible explanation for the presence of ToRSV in the experimental orchard site is that this virus occurred in alternative plant hosts, likely in weeds, prior to the establishment of the pawpaw trees. Then, X. americanum-mediated ToRSV transmission may have occurred from infected weeds to some pawpaw trees and saplings. This hypothesis is further supported by the fact that ToRSV was not identified in symptomatic leaf samples from a pawpaw orchard in Maryland where the virus may not be present. Interestingly, dual infections by TRSV and ToRSV were detected in leaf samples of tree #1 and its sapling, and in flowers of tree #2 . This result may suggest that ToRSV infection in flowers of tree #2 may have resulted from pollen-mediated inoculation. Sequence analyses following HTS and Sanger sequencing of RT-PCR amplicons revealed a close genetic relatedness of TRSV and ToRSV isolates from pawpaw based on a high nucleotide sequence identity in a partial RdRP fragment . A similar high nucleotide sequence identity was found between pawpaw isolates and isolates from other crops . For the partial CP fragment and 30 -UTR of ToRSV, the nucleotide sequence identity was high among isolates from pawpaw but lower between pawpaw isolates and isolates from other crops . Nonetheless, the percent nucleotide sequence identities determined for the ToRSV isolates from pawpaw in the partial RdRP, CP, 25 liter pot and 30 -UTR fragments fall within the ranges previously reported.
Declining pawpaw trees were reported in the late 1990s from Oregon. A disease origin of the decline was suspected but no pathogen, particularly no fungi or bacteria, was consistently isolated from declining trees. Curiously, viruses were not investigated in this work. Our study is the first to ever report the occurrence of a pathogen in symptomatic pawpaw trees with the identification of ToRSV and TRSV. The fact that these two nepoviruses were found in symptomatic trees and some of their saplings was consistent with the tissue connectivity of the tree and its rhizome, explaining the dual infection in the two types of tissues through systemic infection. Similarly, the detection of TRSV in root and fruit tissue of some trees in the pawpaw orchard in Maryland confirms a systemic infection. The recovery from symptoms in pawpaw tree is also consistent with the occurrence of TRSV and ToRSV, two nepoviruses for which initial infection causes severe systemic symptoms and infected plants recover later, as documented on experimental herbaceous plant hosts. The pawpaw is a delicious and nutritious native American fruit. A recent increased interest in pawpaw amongst groups of Indigenous Peoples, gardeners, and nontraditional fruit enthusiasts has driven up the demand for pawpaw fruits and trees. The propagation of clonal pawpaw rootstocks has been unsuccessful; therefore, nurseries currently graft cultivars onto rootstock derived from locally available seeds of diverse genetic origin. Alternatively, nurseries sell dormant or sprouted seeds.
Pawpaw orcharding is advertised as a business opportunity for small growers, particularly as food processors develop recipes to turn mature pawpaw fruit into value-added products. Beyond North America, pawpaw plantings are documented in Romania, Italy, Ukraine, Slovakia, Austria, Georgia, Russia, Japan, and Korea. Some of these plantings are derived from sprouted seeds, whereas others are planted with grafted cuttings. There is no cure for TRSV and ToRSV once trees are infected in the orchard. Therefore, based on the expansion of pawpaw orchards in the United States and worldwide, a careful selection of clean pawpaw seeds and propagation material is critical for producing clean grafted cultivars and preventing the introduction of viruses, such as TRSV and ToRSV, in newly established orchards. Such simple measures are salient for pawpaw because this deciduous fruit tree is considered vulnerable or endangered in several regions of the United States, and there is a strong desire to save this indigenous fruit tree species by Indigenous Peoples in New York.Six to eight pawpaw leaves from the mid-point of branches were collected in October 2021 from symptomatic and asymptomatic pawpaw trees, as well as from symptomatic pawpaw saplings growing underneath the trees in the experimental orchard managed by Cornell University. Leaf samples were similarly collected from two asymptomatic pawpaw trees located on the main campus of Cornell University in Ithaca, New York, which is approximately 12 km away from the experimental orchard.
These samples were selected as negative controls in HTS work to avoid confounding effects of possible virus infections in asymptomatic trees from the experimental orchard. Additional leaf samples were collected from the same pawpaw trees, and eight additional symptomatic and two asymptomatic trees in the experimental orchard in May 2022 for validation work. Finally, flowers were collected from those 12 selected trees in the experimental pawpaw orchard for virus testing. Roots, flowers, small fruits, and symptomatic leaves were also collected from two grafted ‘Shenandoah’ trees in a commercial pawpaw orchard in Westminster, Maryland in June 2022 for virus testing.The goal of all living organisms, as well as plants, is to produce offspring for the next generation. One way plants produce offspring is through making fruits and seeds, the product of successful pollination. Pollination is the process of transferring pollen from the anthers of a flower to the stigma of the same flower or of another one of the same species. Pollen grains of the correct type that land on the stigma germinate and grow a pollen tube down through the style and into the ovules . This process results in fertilization, which, in flowers, leads to the production of fruits and seeds. Seeds contain embryos that become the next generation of plants. Most flowering plants require pollination and have developed a variety of methods for pollen transfer, such as wind, water, biotic agents , and self-pollination. Some flowering plants also reproduce by vegetative means. The relationships that exist between flowering plants and pollinators are not casual, as many plants and pollinators have coevolved over long periods of time to efficiently exchange services. Pollination of flowers by pollinators is the unintended outcome of their activity on the flower. As pollinators visit flowers to sip nectar or gather pollen for their brood, pollen grains are dispersed among plants of the same species, allowing plants to reproduce. Plants have evolved to reward pollinators for visiting their flowers by providing sweet nectar or pollen. The fates of pollinators and flowering plants are thus inextricably bound.Bees are the most important biotic agent for the pollination of agricultural crops, horticultural plants, and wildflowers . People are often most familiar with the introduced European honey bee , which was brought to the eastern United States 400 years ago; it arrived in California around 1850. The honey bee is just one species of bee in this incredibly diverse group of pollinators. Approximately 4,000 species of bees exist in the United States, with 1,600 of those residing in California . About 20,000 species have been recorded worldwide . Native bee species come in a variety of shapes, colors, sizes, and lifestyles that enable them to pollinate a diversity of plant species .Hummingbirds Hummingbirds are the most prominent pollinating birds in North America. These birds have long beaks and tongues that can reach deep into flowers to harvest sugary nectar. When a hummingbird stops in for a sip of nectar, pollen can be transported from the flowers to the beak, feathers, or both. Hummingbirds forage on insects and spiders for their protein source . Butterflies These gorgeous, often colorful, daytime-flying insects have long been purposely lured to flowers by gardeners. Butterfly larvae, or caterpillars, require specific plants to feed on, 25 liter plant pot though the adults can use nectar from many plants.
While adult butterflies are not as efficient pollinators as are bees, they do transfer pollen that sticks to their legs as they flit from flower to flower. Their stunning looks have made them attractions at many botanical gardens and zoos, where butterfly houses and gardens stock their favorite flowers for nectar resources .Moths Moths are mostly evening fliers attracted to many late afternoon, night, and early morning sweet-smelling flowers for their nectar. Moths tend to have hairier and stouter bodies and are generally less colorful than butterflies. They are easy to differentiate from butterflies if you look at their antennae: moths do not have a swelling at the end of their antennae whereas butterflies do. Bats Bats are nocturnal pollinators that play an important role in the pollination of agaves and cacti in the Southwest, as well as many tropical and subtropical plants such as bananas, avocados, and cashews . Bats are usually found visiting light-colored flowers that open at night and often produce copious amounts of pollen and nectar. For example, the Mexican long-nosed bat is a species that has evolved a special head and tongue to allow access to both pollen and nectar from flowers of century plants . Flies Flies are not typically thought of as pollinators; however, research suggests that many flies, such as hover flies and bee flies , are generalist pollinators. Generalist pollinators are important inhabitants of gardens because they visit wide varieties of plants, searching mostly for nectar and in some cases pollen . Beetles Up to 28,000 known beetle species exist in the United States; however, as a group, few species are recognized as pollinators . While beetles are not efficient pollinators, many visit flowers to sip nectar or feed on the flower parts. Their activities may result in pollination. A few plants, such as magnolias, are typically pollinated by beetles.More than 75 percent of flowering plants worldwide rely on animals for pollination; thus ecosystems around the planet are dependent on their services . Pollinators are responsible for the majority of beautiful flowers surrounding us and many of the diverse food plants we eat. In the United States over one-third of the food we eat is dependent on one type of pollinator: bees . If you enjoy watermelon, kiwi, squash, almonds, cherries, peaches, blueberries, apples, and many other fruits, nuts, and vegetables, then you should be concerned about protecting our valuable pollinators. The importance of conserving a diversity of pollinators has become more apparent as the introduced European honey bee population has declined due to colony collapse disorder . Unique to honey bee colonies, CCD causes worker bees to suddenly and mysteriously disappear. Single hive losses ranging from 30 to 100 percent have been reported by beekeepers in both North America and Europe . Honey bees are relatively inexpensive, and the industry standard when experiencing large hive losses is to order additional hives. A growing concern, however, is that if hive losses continue to increase, demand for honey bees will become too great to sustain. What will not decrease is flowering plants’ need for successful pollination from their associated pollinator. While scientists still do not know the exact cause of CCD, they do know that managed honey bees are subjected to a number of stressors, such as pesticide poisoning, low food-source variety, and long transports between various climate regions. These stressors, in addition to predatory mites and pathogens, contribute significantly to honey bee mortality .Properly planned sites in urban areas can provide floral and nesting resources for pollinators as wild land areas decrease. Urban habitat gardens, if planned correctly with appropriate resources, can support pollinator populations . A habitat garden provides wildlife with known resources for survival: food , water , shelter , and a place to raise young . In addition, habitat gardens may see an increase of fruits, nuts, vegetables, and showy flowers . Maximize your yard’s pollinator potential by considering the following recommendations.Seasonal sequence of flowers Plan your garden with a variety of plant types, defined as plant species, subspecies, varieties, cultivars, and hybrids that will bloom from late winter or early spring to late summer or early fall . From 14 years of survey research by the Urban Bee Lab in urban California gardens, it has been determined that a minimum of 20 plant types is necessary to cover the entire bee and flowering seasons from midwinter to the end of October. Selecting plant species with a staggered bloom sequence ensures food for each pollinator’s unique season. As with plants, each pollinator species has its own season of adult activity .