A three year study involving solar heating of soil (soil solarization) with clear polyethylene mulch demonstrated for two years, control of root-knot nematodes (Meloidogyne incognita). The population of M. incognita was reduced >90% in the 0-30cm depth of solarized soil. The number of eggs per gram root recovered and the root gall index from `Georgia-Jet' sweetpotatoes were reduced (92-98%) by soil solarization. Growth and yield were enhanced in solarized soil. The beneficial effects of solarization was observed in the second year following two additional cropping cycles of collard greens and sweetpotatoes.
C. Stevens, V. A. Khan, A. Y. Tang, C. K. Bonsi and M. A. Wilson
Yan Chen, Donald Merhaut and J. Ole Becker
Nitrogen (N) fertilization is critical for successful production of cut flowers in a hydroponic system. In this study, two sunflower cultivars: single-stand `Mezzulah' and multi-stand `Golden Cheer' were grown under two N fertilization rates: 50 mg·L-1 and 100 mg·L-1 in a recirculating hydroponic system. At the same time, `Mezzulah' sunflowers were biologically stressed by exposing each plant to 2000 second-stage juveniles of the plant parasitic nematode Meloidogyne incognita, race 1. The experiment was conducted in May and repeated in Sept. 2004, and plant growth and flower quality between control and nematode-infested plants were compared at the two N rates. The two cultivars responded differently to fertilization treatments. With increasing N rate, the dry weight of `Mezzulah' increased, while that of `Golden Cheer' decreased. Flower size and harvest time were significantly different between the two cultivars. However, N had no effect on flower quality and harvest time. Flower quality rating suggests that quality cut stems can be obtained with 50 mg·L-1 N nutrient solution. Nematode egg count suggests that plants in the nematode treatment were successfully infested with Meloidogyne incognita, however, no significant root galling was observed, and plant growth and flower quality were not affected by nematode infestation.
Thirteen sweetpotato (Ipomoea batatas) genotypes were characterized for resistance to Meloidogyne incognita, M. javanica, M. hapla, and M. arenaria races 1 and 2 in greenhouse tests. The following sweetpotato genotypes representing a range of reactions to M. incognita were evaluated: U.S. Plant Introduction (PI) 399163 (highly resistant = HR), Sumor (HR), Nemagold (HR), Excel (HR), Tinian (HR), Hernandez (resistant = R), Jewel (R), Regal (R), Porto Rico (intermediate = I), Centennial (susceptible = S), Georgia Jet (S), Sulfur (S), and Beauregard (S). Meloidogyne incognita was most pathogenic to sweetpotato of the four Meloidogyne spp. evaluated in these studies. The U.S. Plant Introduction (PI) 399163 and Sumor were resistant to M. incognita in all tests. Only two genotypes, Beauregard and Porto Rico, were susceptible to M. javanica. All genotypes evaluated were resistant to M. hapla, M. arenaria race 1, and M. arenaria race 2. Sumor, U.S. PI 399163, and Nemagold appear to provide the highest levels of resistance against the four Meloidogyne spp. used in these studies. Since M. incognita is the most commonly occurring root-knot nematode species in sweetpotato growing areas of the southern U.S. and is pathogenic to most of the commonly grown sweetpotato cultivars, efforts to develop resistant cultivars that have desirable horticultural characteristics for the U.S. market should be directed toward this root-knot nematode species.
Luisa Santamaria and Sherry Kitto
Solanum quitoense, also known as naranjilla or lulo, is a native species of Ecuador and Colombia. Its value is based on the uncommon sweet-sour flavor of its fruits, which is appreciated in the national and international markets. The worst problem for this crop is the root-knot nematode Meloidogyne incognita. The main objective of our research is to develop root-knot nematode–resistant naranjilla via somaclonal variation. Seeds of Solanum quitoense `Baeza' germinated quicker than those of `Dulce'. Seeds given a 2-week dark treatment had 100% germination compared to 75% germination for seeds placed under lights (16-h photoperiod, 60 mmol·m–2s–1). Single-node explants proliferated an average of nine nodes after 1 month of culture. Microcuttings (two nodes, 3.5 cm) stuck in sand and placed under a humidity dome under mist had an average of five roots averaging 25 cm in length after 3 weeks. Stems regenerated shoots better than petioles or leaves and explant orientation/polarity had no effect on regeneration. Root cultures of Solanum quitoense inoculated in vitro with Meloidogyne incognita showed susceptibility to root-knot nematodes.
Mwamburi Mcharo*, Don Labonte, Chris Clark and Mary Hoy
Using two sweetpotato (Ipomoea batatas (L.) Lam) F1 populations from diverse environments we investigated the AFLP marker profiles of the genotypes for association studies between the molecular markers and southern root-knot nematode (Meloidogyne incognita) resistance expression. Population one consisted of 51 half-sib genotypes developed at the Louisiana State Univ. AgCenter. The second population consisted of 51 full-sibs developed by the East African and International Potato Center sweetpotato breeding programs. Results for nematode resistance expression indicate a binomial distribution among the genotypes. Using analysis of molecular variance, logistic regression and discriminant analysis, AFLP markers that are most influential with respect to the phenotypic trait expression were selected for both populations. A comparative analysis of the power of models from the two statistical models for southern root-knot nematode resistance class prediction was also done. The diversity and possible universal similarity of influential markers between the two populations and the expected impact in sweetpotato breeding programs will be discussed.
Nancy Kokalis-Burelle, C.S. Vavrina, M.S. Reddy and J.W. Kloepper
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.
P. D. Dukes and Janice R. Bohac
There are four known physiological races of the southern root-knot nematode [Meloidogyne incognita (Kofoid & White) Chitwood]. Races are designated I through 4 and their identifications are based soley on differential hosts. These race problems as related to breeding sweetpotato for resistance to attack by all races are reviewed and discussed. Data are presented showing the reactions of selected cultivar and breeding clones of sweetpotato to all four races. The reactions of races 1 and 3 are generally well—known. Races 2 and 4 apparently are spreading and becoming more numerous in the southern states where soybean and tobacco are grown. Comparative disease indices are presented showing that generally sweetpotatoes were less susceptible to races 2 and 4. However, there were some notable exceptions, for example, `Sulfur' and `Beauregard' were equally susceptible to all races. High resistances to attack by races 2 and 4 were found in `Sumor', `Nemagold', `Excel', W-241 and others.
R. Mark Hurley, Paul G. Thompson and Gary W. Lawrence
A factorial test was conducted to evaluate the potential of screening sweetpotato plants to three pathogens simultaneously. The pathogens were Meloidogyne incognita, Fusarium oxysporum, and Streptomyces ipomoea. The test also involved six sweetpotato cultivars and three evaluation times. Evaluation times were 3, 6, and 9 weeks post inoculation. The symptoms evaluated were vascular necrosis, fibrous root necrosis, and gall and egg mass production. For each of the three pathogens, the ability to separate cultivars with intermediate levels of resistance from those with low levels of resistance decreased as post Inoculation time increased. Simultaneous screening was practical if the goal was to select plants with resistance to all three pathogens. Resistances to individual pathogens could not be identified in plants inoculated with all three pathogens.
F.A. Hammerschlag and R.N. Huettel
Five in vitro propagated peach scion cultivars (Suncrest, Rio Oso Gem, Compact Redhaven, Redhaven, Jerseyqueen) and two rootstock (Nemaguard and Lovell) were screened in vitro and in microplots for their susceptibility to the root-knot nematode, Meloidogyne incognita. Evaluations in tissue culture for galling were conducted at 5 wk. Trees in microplots were evaluated for 3 years for nematode populations, trunk diameter, and yield. Comparative results indicated that the number and size of galls observed at 5 wk in vitro is indicative of the response of peaches to nematodes under field conditions after three years. Cultivar Compact Redhaven was significantly more tolerant to root-knot than `Lovell' the most widely used peach rootstock. These results suggest that Compact Redhaven might be potentially useful as a rootstock in the Southeast where Nemaguard is used sparingly because of its lack of cold tolerance. In addition, these results indicate that in vitro screening holds promise as a rapid technique for evaluating root-knot nematode resistance.
Howard F. Harrison, Judy A. Thies, Richard L. Fery and J. Powell Smith
A preliminary screening experiment was conducted to evaluate 47 cowpea [Vigna unguiculata, (L.) Walp.] genotypes for use as a weed-suppressing cover crop. Lines evaluated in this study included forage varieties, PI accessions, experimental breeding lines, and land races of unknown origin. Of these, 11 were selected for further testing on the basis of vigorous growth and weed-suppressing ability. In a field experiment repeated over 4 years, the selected genotypes were not different from the leading cover crop cultivar, `Iron Clay', in biomass production. Vigor ratings, vine growth ratings, and canopy widths of some genotypes exceeded those of `Iron Clay'. Vigor ratings and canopy measurements were efficient selection criteria that could be useful for breeding cover crop cowpea cultivars. All selections except an African cultivar, `Lalita', were highly resistant to southern root knot nematode [Meloidogyne incognita (Kofoid and White) Chitwood], and the genotypes varied in seed size, photoperiod, and response to diseases.