Phytophthora capsici fruit rot is an increasingly serious disease affecting cucumber production throughout the Eastern U.S. The absence of genetically resistant cultivars and rapid development of fungicide resistance makes it imperative to develop integrated disease management strategies. Cucumber fruits which come in direct contact with the soil-borne pathogen are usually located under the canopy where moist, warm conditions favor disease development. We sought to examine whether variations in plant architecture traits that influence canopy structure or fruit contact with the soil make conditions less favorable for disease development. As a `proof of concept' to test whether an altered canopy could facilitate P. capsici control, we tested the effect of increased row spacing and trellis culture on disease occurrence in the pickling cucumber `Vlaspik.' Trellis plots indicated that removal of fruit contact from soil reduced disease occurrence. Currently available variation in plant architecture was tested using nearly-isogenic genotypes varying for indeterminate (De), determinate (de), standard leaf (LL), and little leaf (ll) traits. Although differences were observed in peak mid-day temperatures under the different canopies, there were not differences in disease occurrence among the genotypes. A collection of 150 diverse cucumber accessions identified to serve as a representative sample of the germplasm, was observed for possible variation in plant architecture. Variation was observed for an array of traits including main stem length, internode length, leaf length and width, and number of branches. Interesting types that may allow for more open canopies include reduced branching habit and compact/bushy growth.
Kaori Ando* and Rebecca Grumet
Kaori Ando and Rebecca Grumet
Fruit development proceeds from cell division to expansion, maturation, and ripening. Expansion is critical for size, yield, and quality; however, this period of development has received little attention. We used 454-pyrosequencing to develop a cucumber (Cucumis sativus) fruit transcriptome, identify highly expressed transcripts, and characterize key functions during exponential fruit growth. The resulting 187,406 expressed sequence tags (ESTs) were assembled into 13,878 contigs. Quantitative real-time polymerase chain reaction (qRT-PCR) verification of differentially expressed genes from fruit of different ages, and high correlation in transcript frequency between replicates, indicated that number of reads/contig reflects transcript abundance. Putative homologs were identified in Arabidopsis thaliana for 89% of the contigs represented by at least 10 ESTs; another 4% had homologs in other species. The remainder had homologs only in cucurbit species. The most highly expressed contigs were strongly enriched for growth (aquaporins, vacuolar ATPase, phloem proteins, tubulins, actins, cell wall-associated, and hormone-related), lipid, latex, and defense-related homologs. These results provide a resource for gene expression analysis in cucumber, profile gene expression in rapidly growing fruit, and shed insight into an important, but poorly characterized, developmental stage influencing fruit yield and quality.
Kaori Ando and Rebecca Grumet
Fruit rot induced by Phytophthora capsici Leonian is an increasingly serious disease affecting pickling cucumber (Cucumis sativus L.) production in many parts of the United States. The absence of genetically resistant cultivars and rapid development of fungicide resistance makes it imperative to develop integrated disease management strategies. Cucumber fruit which come in direct contact with the soil-borne pathogen are usually located under the canopy where moist and warm conditions favor disease development. We sought to examine whether variations in plant architecture traits that influence canopy structure or fruit contact with the soil could make conditions less favorable for disease development. As an extreme test for whether an altered canopy could facilitate P. capsici control, we tested the effect of increased row spacing and trellis culture on disease occurrence in the pickling cucumber `Vlaspik'. Temperature under the canopy was lowest in trellis plots, intermediate in increased spacing plots, and highest in control plots. Disease occurrence in the trellis plots was significantly lower than in other treatments, indicating that preventing fruit contact with the soil reduced disease occurrence. The effect of currently available variation in plant architecture was tested using nearly-isogenic genotypes varying for indeterminate (De), determinate (de), standard leaf (LL), and little leaf (ll) traits. Plants with standard architecture had higher peak mid-day temperatures under the canopy and greater levels of P. capsici infection; however, levels of disease occurrence were high for all genotypes. Screening a collection of ≈150 diverse cucumber accessions identified to serve as a representative sample of the germplasm, revealed variation for an array of architectural traits including main stem length, internode length, leaf length and width, and number of branches; values for `Vlaspik' were in the middle of the distribution. Plant architectures that may allow for more open canopies, including reduced branching habit and compact growth, were tested for disease incidence. One of the compact lines (PI 308916), which had a tendency to hold young fruit off the ground, exhibited lower disease occurrence. The reduced disease occurrence was not due to genetic resistance, suggesting that architecture which allows less contact of fruit with the soil could be useful for P. capsici control for pickling cucumber.
Kaori Ando and Rebecca Grumet
Phytophthoracapsici, a soil-borne oomycete pathogen causing fruit rot in cucumber, has become a limiting factor for cucumber production in the Midwest. In the process of screening plant introductions (PIs) for resistance to P. capsici, it appeared that degree of susceptibility might decrease as fruits develop. To examine this more carefully, detached, greenhouse-grown, hand-pollinated `Vlaspik' fruits aged 2–18 days post-pollination (dpp) were inoculated with P. capsici mycelium and evaluated for symptoms. There was a reproducible decrease in susceptibility with increasing fruit age. The fruits that sporulated were usually younger and smaller (2–9 dpp), 10-3 dpp fruit tended to develop water-soaked symptoms, while the fruits that remained symptom-free were usually older (>14 dpp) and oversized for pickling cucumber. The transition from susceptible to more resistant appeared to occur at the end of the period of rapid fruit elongation. Detached field-grown `Straight Eight' fruits showed similar size-related trends. Candidate resistant genotypes identified from the PI screening were re-screened using 7 and 14 dpp fruits. Again an age-dependent difference in response was observed, indicating that the increase in resistance is not genotype-specific. Furthermore, field observations suggest a gradation of susceptibility within the fruits as the blossom end was most frequently infected. Preliminary tests of detached greenhouse-grown, hand-pollinated fruits suggested that as the fruits grew older, the blossom end remained susceptible longer than the stem end. These findings could have implications for appropriate screening methods, the stage of fruit likely to become infected in the field, and appropriate spray practices.
Kaori Ando, Sue Hammar, and Rebecca Grumet
Phytophthora capsici causes severe losses in vegetable production, including many cucurbit crops. Our previous work showed that cucumber (Cucumis sativus) fruit are most susceptible to P. capsici when they are very young and rapidly elongating, but develop resistance as they approach full length at 10 to 12 days postpollination (DPP). In this study, fruit from seven additional cucurbit crops representing four species, melon (Cucumis melo), butternut squash (Cucurbita moschata), watermelon (Citrullus lanatus), and zucchini, yellow summer squash, acorn squash, and pumpkin (Cucurbita pepo), were tested for the effect of fruit development on susceptibility to P. capsici. Field-grown fruit of the different crops varied in overall susceptibility. Zucchini and yellow summer squash were the most susceptible, with the majority of fruit exhibiting water-soaking symptoms within 24 hours postinoculation. Fruit from all of the crops exhibited size-related decrease in susceptibility, but to varying degrees. Cucumber had the most pronounced effect. In infested fields, cucumber fruit were found to be most frequently infected at the blossom end. Comparison of the peduncle and blossom end showed a difference in susceptibility along the length of the fruit for cucumber, butternut squash, and zucchini. Greenhouse-grown, hand-pollinated pumpkin, acorn squash, and butternut squash showed an age-related decrease in susceptibility similar to field-grown fruit. For all of these fruit, a pronounced reduction in susceptibility accompanied the transition from the waxy green to green stage at ≈3 to 8 DPP.
Laura Havenga, Kaori Ando, and Rebecca Grumet
Cucumber PI308916 has a compact growth habit (short internodes and main stem length) due to a single recessive gene cp. We also observed that this PI has lower incidence of Phytophthora fruit rot, likely due to its upright fruit bearing habit. Previous researchers reported that compact cucumber lines derived from PI308916 exhibited the potential for higher yields due to increased planting density. Despite these potentially beneficial traits, efforts to develop PI308916-derived lines were terminated due to poor seedling establishment. The objective of this research is to determine the relationship between the compact phenotype and poor seedling establishment. Short internodes can be caused by deficiency in gibberellins or brassinosteroids that can also impact germination or apical hook formation, a trait important for seedling emergence from the soil. Germination rate and apical hook angle was recorded for `Wautoma' (control inbred line that forms normal apical hook), PI308916, and their F1, F2, and BC progeny. Germinating `Wautoma' seeds showed consistent, large apical hook angles (mean 147), while hook angles of PI308916 were broadly distributed from 0 to 180 (mean 96). F1 progeny for the reciprocal crosses had a similar angle (mean 134 and 133) to `Wautoma'. Segregation ratios for apical hook angle in the F2 and BC populations were consistent with a single recessive gene. Evaluation of the relationship between the apical hook and compact phenotype, showed an absence of the recombinant class of long internodes and small apical hook angle in the F2 population, suggesting that the two traits may be conferred by a single gene or two tightly linked genes.
Callista Ransom, Chrislyn Particka, Kaori Ando, and Jim Olmstead
Prabin Tamang, Kaori Ando, William M. Wintermantel, and James D. McCreight
Cucurbit yellow stunting disorder virus (CYSDV) is a devastating viral disease of melon that can cause significant yield and quality losses. This disease has recently emerged as a major concern in the southwest United States and major melon-growing regions across the world. Coinfection of melon by Cucurbit chlorotic yellows virus (CCYV) was recognized in Imperial Valley and neighboring production areas of California and Arizona in 2018, but its importance remains largely unknown. Identifying and deploying CYSDV resistance from elite germplasm is an economical and effective way to manage the disease. A F2:3 population was developed from a cross of susceptible ‘Top Mark’ with CYSDV-resistant PI 313970, which was shown to possess a single recessive gene for resistance to CYSDV. The F2:3 population was phenotyped in the field in response to natural, mixed infections by the two viruses, CYSDV and CCYV in the Fall melon seasons of 2018 and 2019. Phenotypic data (foliar yellowing) from both years were not useful for mapping CYSDV resistance quantitative trait loci (QTL), as PI 313970 and CYSDV-resistant F2:3 plants exhibited yellowing symptoms from CCYV coinfection. QTL analysis of the relative titer of CYSDV calculated from reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) data identified one locus on chromosome 3 at the physical location of S5-28,571,859 bp that explained 20% of virus titer variation in 2018 but was undetected in 2019. A locus on chromosome 5 between S5-20,880,639 to S5-22,217,535 bp explained 16% and 35% of the variation in CYSDV titer in 2018 and 2019, respectively. One or both of the markers were present in six of 10 putative melon CYSDV resistance sources. Markers flanking the 2019 QTL were developed and can be used in marker-assisted breeding of CYSDV-resistant melons.
Adam D. Call, Adam D. Criswell, Todd C. Wehner, Kaori Ando, and Rebecca Grumet
Downy mildew, a foliar disease caused by the oomycete Pseudoperonospora cubensis (Berk. and Curt.) Rostow, is one of the most destructive pathogens of cucurbits. From 1961 to 2003, resistant cucumber cultivars in the United States had sufficient resistance to grow a successful crop without the use of fungicides. The pathogen resurged as a major problem in 2004. Since then, the dm-1 gene has not been effective against the new strain of downy mildew, and yield losses are high without the use of fungicides. The objective of this experiment was to identify cultivars having high yield and resistance to the new downy mildew. The experiment had 86 cultivars and breeding lines (hereafter collectively referred to as cultigens) and was conducted in Clinton, NC, in 2007 and 2009, in Castle Hayne, NC, in 2008 and 2009, and in Bath, MI, in 2007 to 2009. Plots were rated weekly on a 0 to 9 scale (0 = none, 1–2 = trace, 3–4 = slight, 5–6 = moderate, 7–8 = severe, and 9 = dead). Mean ratings for downy mildew leaf damage ranged from 2.9 to 5.7 in Michigan in 2008 and 2009 and from 3.8 to 6.8 in North Carolina in 2007 to 2009. None of the cultigens tested in this study showed a high level of resistance, although differences in resistance were detected. Lines WI 2757 and M 21 and cultivar Picklet were consistently among the top resistant lines in North Carolina and Michigan. The cultivars Coolgreen, Wis. SMR 18, and Straight 8 were identified as moderately to highly susceptible. An unreleased hybrid, ‘Nun 5053 F1’, and the cultivar Cates were the top yielding lines overall. The highest yield in a single year and location was from the cultivar Cates in Clinton, NC, in 2009, with 25.6 Mg·ha−1. The best cultivars in this study were only moderately resistant and would likely require fungicide applications to achieve high yield and quality in the presence of downy mildew. Until high resistance becomes available, growers would benefit by using fungicides in combination with tolerant and moderately resistant cultigens.