Greenhouse and field trials were performed on muskmelon (Cucumis melo) and watermelon (Citrullus lanatus) to evaluate the effects of six formulations of plant growth-promoting rhizobacteria (PGPR) that have previously been shown to increase seedling growth and induce disease resistance on other transplanted vegetables. Formulations of Gram-positive bacterial strains were added to a soilless, peat-based transplant medium before seeding. Several PGPR treatments significantly increased shoot weight, shoot length, and stem diameter of muskmelon and watermelon seedlings and transplants. Root weight of muskmelon seedlings was also increased by PGPR treatment. On watermelon, four PGPR treatments reduced angular leaf spot lesions caused by Pseudomonas syringae pv. lachrymans, and gummy stem blight, caused by Didymella bryoniae, compared to the nontreated and formulation carrier controls. One PGPR treatment reduced angular leaf spot lesions on muskmelon compared to the nontreated and carrier controls. On muskmelon in the field, one PGPR treatment reduced root-knot nematode (Meloidogyne incognita) disease severity compared to all control treatments.
Nancy Kokalis-Burelle, C.S. Vavrina, M.S. Reddy and J.W. Kloepper
J.M. Sherman, K.S. Reddy, S.E. Newman and J.A. Spencer
Most modern roses are highly susceptible to the disease blackspot caused by the fungus Diplocarpon rosae. This contrasts to species roses that are resistant to the disease. To gain information on the biochemical factors in this resistance mechanism, we are studying the involvement of proteins. Soluble proteins of modern roses and species roses were extracted and analyzed by SDS-PAGE. When healthy leaves were examined, there were no distinct differences in the protein patterns, indicating that there are no constitutive proteins involved in the disease resistance mechanism. There were no differences between healthy and infected leaves of resistant genotypes. When detached leaves of some susceptible types were infected with the fungus new proteins seemed to appear in the healthy region surrounding the blackspot lesion. These proteins may be involved in resisting the spread of the pathogen.
J. M. Sherman, K. S. Reddy, S. E. Newman and J. A. Spencer
The objective of this study was to determine if blackspot (Diplocarpon rosae, Wolf) resistance and susceptibility is expressed in callus derived from known resistant and susceptible genotypes of roses. Established callus lines of blackspot susceptible hybrid tea roses `Pascali' and `Tropicana' and blackspot resistant species roses Rosa roxburghii and R. setigera were inoculated directly with a blackspot conidia suspension. Uninoculated callus served as controls. Cultures were incubated at 25°C for 28 days in the dark. An evaluation of the appearance and precent change in fresh weight indicated that for all genotypes tested, inoculation resulted in a decline in tissue appearance and rate of weight increase. These results indicated that in vitro. resistance to blackspot differs from the resistance found in whole plants.
J.M. Sherman, K.S. Reddy, S.E. Newman and J.A. Spencer
The objective of this study was to determine whether tissue culture can be used for studying the blackspot resistance found in some roses. Callus was initiated from leaves, petioles, and stems of resistant and susceptible genotypes. Good callus formation for susceptible roses (hybrid teas) was obtained on a medium containing MS basal salts, vitamins, sucrose, and 8 g/L agar supplemented with 2 mg/L 2,4-D, 1 mg/L NAA, and 0.2 mg/L BA. Callus formation for resistant roses (species roses) was best when the concentrate ions of growth regulators in the medium were halved. Browning in species rose cultures, was decreased with the addition of 0.05 g/L ascorbic acid to the medium followed by incubation in the dark. The subculture calli were inoculated with the fungal conidia and were analyzed for proteins by SDS-PAGE. These protein profiles were compared to those of whole leaf samples. The results are discussed in terms of similarities and differences in the biochemical responses of callus cultures versus whole leaves to the blackspot infection.
M.J. Iqbal, L.J. Grauke, A.S. Reddy and T.E. Thompson
A microsatellite library has been developed from `Halbert', a native pecan selection from Coleman County, Texas, using methods developed at the Texas A&M Univ. Crop Biotechnology Center. A total of 6144 DNA fragment clones were archived in 384 well plates for screening. Four-hundred-thirty-nine clones were positive after Southern hybridization using di- and tri-nucleotide repeats as probes. One-hundred-twenty-five positive clones were sequenced on an ABI 377 automated DNA sequencer. Of these, 24 repeats had enough sequences at the two ends to design primers. Primers were designed using Primer Express software, and were synthesized by Genosys, USA. The simple sequence repeats (SSRs) chosen for primer analysis include di- (CA and GA) and tri-nucleotide repeats (CTT, GAA and GAT). The SSRs were amplified under high stringency conditions with temperatures based on length and GC content. Reproducibility was verified using `Halbert' DNA isolated from different inventories. Of the 24 primer pairs tested, 20 successfully amplified microsatellites from `Halbert'. DNA was isolated from 48 pecan and hickory accessions selected to strategically represent the genetic diversity of the NCGR Carya collections (a core collection). The accessions included parent-progeny combinations, individuals from geographically distant native populations, species, and interspecific hybrids. The 20 SSR primers that produced good amplification products in `Halbert' were used to evaluate the collection, with 11 revealing multiple sizes of the repeat. The number of bands amplified with different primer combinations ranged from 4 to 32 in the 48 genotypes tested. We used RFLPscan software to aid in gel scoring (sizing amplified fragments, and comparing amplification profiles), and NTSYSpc software to evaluate genetic similarities. Evaluation of the data confirms the utility of the primers in delimiting known relationships.
Joseph W. Kloepper, M.S. Reddy, Choon-min Ryu and John F. Murphy
Use of beneficial rhizobacteria to enhance growth and induce systemic disease protection in transplants. Plant associated bacteria have been studied for the capacity to provide plant growth enhancement and biological disease control. “Rhizobacteria” are bacteria from the rhizosphere that have the capacity to colonize plant roots following introduction onto seeds or into soil. Effects of rhizobacteria on plants may be deleterious, neutral, or beneficial. Beneficial rhizobacteria are termed “PGPR—plant growth-promoting rhizobacteria.” In developmental studies aimed at reducing to practice the concept of induced systemic disease protection mediated by PGPR, we discovered that mixtures of PGPR and an organic amendment into the soilless media used to prepare tomato transplants resulted in highly significant and reproducible plant growth promotion. Time for development of transplants was typically reduced from 6 weeks for controls receiving industry standard fertility and growth regimes to 4 weeks for seedlings grown in soilless mix into which the PGPR had been incorporated. This marked growth promotion was also associated with systemic protection against pathogens. When transplants were inoculated with the tomato spot pathogen, significantly fewer lesions developed on plants grown in the biological system than on control plants. Similar effects on plant growth and systemic disease protection were seen with cucumber, bell pepper, and tobacco, suggesting that the benefits are not highly crop or cultivar specific. Results of recent field studies will be presented. We conclude that incorporation of PGPR into soilless mixes is a technologically useful and feasible way to deliver benefits to transplants.