Individual weed biomass for A. palmeri and D. sanguinalis, however, was lower for all weed densities when grown in the presence of sweet potato compared with weeds grown without sweet potato. The reduced individual biomass and biomass per meter of row for both weeds, when grown with sweet potato, indicate that interspecific interference is occurring between sweet potato and weeds. Crop biomass reductions are generally associated with increased weed competition and yield losses . However, in this study, although weed biomass was lower when grown with sweet potato, increased weed density did not reduce sweet potato biomass, despite the reduction in sweet potato yield at the same densities.Individual dry biomass of each weed species growing without sweet potato decreased as weed density increased . In the absence of sweet potato, individual dry biomass of both weeds was fit to a linear-plateau model. Individual weed biomass was greatest for both weeds at the lowest density. Amaranthus palmeri and D. sanguinalis individual plant biomass decreased 71% from 1 to 3 plants m−1 of row and 62% from 1 to 4 plants m−1 of row, respectively, and remained unchanged at densities above 4 plants m−1 row for both weeds . This finding was similar to the trend observed in peanut for A. palmeri. We believe that the reduction in individual weed biomass for A. palmeri and D. sanguinalis at lower weed densities when grown without sweet potato is due to increasing intraspecific competition as weed density increases. At the higher densities of both weeds,dutch bucket hydroponic the impact of intraspecific competition has limited effect on further decreasing individual weed biomass. The established threshold is the density at which all weeds achieve maximum accumulated biomass before intraspecific competition begins.

Further biomass increases would require densities resulting in weed mortality due to intraspecies competition, and such densities were not evaluated in this study. This study demonstrates that A. palmeri and D. sanguinalis have the ability to reduce yield at densities as low as 1 to 2 plants m−1 row. Sweetpotato competes with A. palmeri or D. sanguinalis, resulting in reduced weed biomass. This observation suggests that sweet potato with rapid canopy establishment and dense growth habit may provide additional competition with weeds and reduce yield loss, as proposed by Harrison and Jackson . Future studies should establish critical weed-free periods for these weeds in sweet potato, investigate competitiveness of resistant weed biotypes with sweet potato, and determine weed interference with sweet potato under varying management practices .The Salinas and Pajaro Valleys of coastal central California are among the most important lettuce-producing regions in the United States. One of the top disease concerns for lettuce in the area is Verticillium wilt caused by the fungus Verticillium dahliae, which is a soilborne pathogen with a wide host range that also includes artichoke, cotton, eggplant, hops, potato, sunflower, tobacco, and tomato. Two races of V. dahliae occur in coastal central California based on their differential virulence on cultivar La Brillante; however, race 1 is more prevalent and economically important than race 2. In tomato, race 1 of V. dahliae carries Ave1 that is recognized by Ve1 in resistant genotypes. Ve genes encode receptor-like proteins with extracellular leucine-rich repeats; such RLPs are widespread in land plants. In addition to Ve1, tomato also contains the closely linked paralog Ve2; their encoded RLPs work antagonistically to confer resistance to V. dahliae race 1. Several Ve paralogs also confer resistance in otherwise V. dahliae-susceptible species including cotton, potato, hops, and wild eggplant, but it is unknown whether they function analogously to the tomato Ve genes in conferring V. dahliae race 1resistance. In lettuce, resistance to V. dahliae race 1 was originally identified in the Batavia-type cultivar, La Brillante, as conferred by a single dominant locus located on chromosomal linkage group 9. The lettuce Vr1 locus contains several genes with sequence similarity to the Ve genes of tomato; it is very likely that one or more of these LsVe homologs are functional resistance genes.

The goals of this study were to identify the lettuce Ve allele that play a role in resistance to V. dahliae race 1 and to develop PCR-based assays for marker-assisted selection. For this purpose, we analyzed the genome sequences of cultivars La Brillante and the previously published Salinas. Subsequently, we sequenced and/or used allele-specific PCR screens of 150 additional lettuce accessions to identify the allele of the LsVe genes that are exclusively present in resistant phenotypes.Cultivar La Brillante is a Batavia type lettuce with a small, round head that is less dense than those of modern iceberg cultivars. Because of the certain phenotypic similarities in the shape of heads, fewer back crosses are usually needed to develop true to type iceberg cultivars when introgressing desirable genes from Batavia accessions than would be needed if those genes were introgressed from non-heading types of lettuces. Our current analyses showed that besides cultivar La Brillante, another Batavia cultivar can also be used for a relatively rapid development of iceberg cultivars with resistance to V. dahliae race 1. Both of these cultivars contain the same combination of LsVe alleles . Only two out of 36 romaine accessions were resistant to the disease in field experiments. One of the resistant accessions, cultivar Annapolis, is a dark red lettuce with a relatively small and light head that is usually grown for baby leaf production and is therefore harvested at early stages of development. The other resistant cultivar was Defender, which is green. Origin of resistance in this cultivar is unknown because it was developed through open pollination. A high frequency of resistance to the disease was found in Latin type accessions thatphenotypically resemble a small romaine lettuce with more pliable and oily leaves. Because of the phenotypic similarity between romaine and Latin types, Latin-type accessions may also be used for a relatively rapid development of romaine cultivars with resistance to V. dahliae race 1. Both romaine cultivars and three sequenced Latin cultivars that are resistant to the disease contain an identical combination of LsVe alleles . Substantially different frequencies of LsVe1L alleles and resistant phenotypes in different horticultural types of lettuce are not unexpected considering that comparable differences were previously described for other monogenically inherited traits, such as resistance to lettuce dieback and sensitivity to triforine.

Differences in the frequency of specific alleles among horticultural types are likely caused by the breeding approach that is used to develop lettuce cultivars. Only a few elite progenitors or founder lettuce cultivars have given rise to most of the modern commercial cultivars. Each of these progenitors is frequently found in pedigrees of cultivars of the same horticultural type. Additionally, new cultivars are mainly developed by recurrent breeding within small pools of closely related germplasm of the same type. Therefore, alleles present in an original progenitor of a certain type are found in high frequency in cultivars of the same type, but may be absent or present in low frequency in cultivars of other horticultural types. Our data are consistent with the LsVe1 gene identified in the cultivar La Brillante being involved in resistance to V. dahliae race 1 in lettuce. Among the 152 accessions included in this study, 21 were resistant to V. dahliae race 1 and all 21 contained the LsVe1L allele; this allele was not present in any of the susceptible accessions. The other La Brillante Ve alleles, LsVe3L and LsVe4L,blueberry grow pots were also present in all the resistant accessions, but they also occurred in two and twelve susceptible accessions, respectively. Therefore, LsVe1L is the strongest candidate as being required for resistance to V. dahliae race 1 in lettuce, although our data do not exclude LsVe3L or LsVe4L from also being involved similarly as in tomato. Complementation and knock-out studies are still required to determine the functional basis of LsVe-mediated resistance to V. dahliae race 1. The function and the significance of the differences between the LsVe1L and LsVe1S alleles remains to be investigated. The proteins encoded by LsVe1L and LsVe1S have the same domain organization, including the 37 extracellular, leucine-rich repeats separated by a short spacer region, as in previously characterized functional Ve proteins in other species. However, in addition to sequence diversity in the extracellular LRR domain, LsVe1L has an additional Cterminal transmembrane domain as compared to Ve1 and Ve2 in tomato, suggesting that maybe LsVe1L crosses the membrane three times and terminates with anon-cytoplasmic domain instead of a cytoplasmic domain. The distribution of disease incidence in susceptible accessions and across horticultural types indicates a possible presence of a modifying factor or factors that affect disease incidence. Our data do not exclude the possibility of interactions between two or more Ve genes, similar to those reported in tomato. A more detailed study of accessions with different frequencies of disease incidence and allelic compositions is needed to elucidate the basis of variation in disease incidence.Experiments were conducted in a field infested with V. dahliae race 1 located at the USDA-ARS station in Salinas, California. One hundred and fifty accessions were direct-seeded in a randomized complete block design with three replications. The original seed batches of previously sequenced cultivars Salinas and La Brillante were not available for field tests; therefore, seed batches used in field tests are shown as separate entries . Each plot was 7 m long and consisted of two seed lines on 1 m wide beds standard for lettuce production in coastal California.

Plant spacing was approximately 28 cm between seed lines and 30 cm between plants within a seed line. All field experiments were maintained using standard cultural practices for coastal California lettuce production.Unless indicated otherwise, ten plants from each plot were uprooted and visually evaluated. Disease incidence was assessed by cutting taproots longitudinally and recording the number of plants exhibiting the yellowish-brown discoloration of root vascular tissues that is typical of Verticillium wilt. Absence of V. dahliae race 1 in cultivars with race 1-resistant genotype was confirmed by plating surface-sterilized symptomatic root tissue on NP-10 semi-selective agar medium and PCR screening any resulting isolates with Ave1-specific primers. Three additional experiments were performed in the same field to confirm phenotypic observations. These experiments comprised only a subset of accessions that were either symptomless in the first experiment or were used as susceptible checks. Disease incidence values from all four experiments were combined and used for statistical analyses with JMP 14.2 .Intracellular transport of plant viruses during infection operates via unique interactions between viral proteins and selected host cytoskeletal and membrane elements. Microtubules and/or actin filaments are required for host cytoskeleton functions that are involved in virus replication and movement . Virus infections often result in modification of these systems and mechanisms affecting these processes have become special topics of enquiry over the past decade . For example, the roles of cellular remodeling in formation of replication factories and intracellular movement of several viruses has been reviewed recently . Since these reviews, the Potato virus X triple gene block 1 movement protein has been shown to remodel actin and endomembranes to form X-bodies that function in replication, and to recruit the TGB2 and TGB3 proteins to the X-bodies . Also, the endoplasmic reticulum and Golgi apparatus have been shown to be extensively remodeled as a consequence of Turnip mosaic virus infection . Modified actin filaments and the endomembrane system are involved intracellular and intercellular movement of several plant viruses . The plasma membrane and ER are continuous between cells, and form desmotubule conduits for intercellular movement of macromolecules between adjacentcells that are regulated by actin filaments, myosin-motor associations, and/or poorly understood membrane flow mechanisms . Virus studies over the past decade have shown that plasmodesmata connections forming cellular symplast boundaries are remarkably plastic and are involved in numerous complex interactions required for virus transit . Considerable variation is now apparent in mechanisms of virus intercellular movement, and this is most evident in viruses in which nucleoprotein complexes move through enlarged PD, versus viruses in which remodeled tubular PD function in cell-to-cell transit of whole virions . All plant viruses encode movement proteins that function in cell-to-cell movement, but these proteins and the pathways involved in movement vary enormously in their complexity and host component interactions . The most intensively studied MPs are Tobacco mosaic virus 30K protein, TGB proteins encoded by hordeiviruses and potexviruses, the Closterovirus movement complex proteins, the Nepovirus 2B protein, and the P6 Cauliflower mosaic virus . Each of these MP complexes interact in a variety of ways with endomembrane components, cytoskeleton networks and PD .