Root-knot nematode infection of pecan trees results in reduced growth and vigor of above ground plant parts and gall formation on roots. One-year-old roots parasitized by nematodes were collected and examined by light and scanning electron microscopy for structural anomalies. The root's injury response to nematode feeding was dependent on the location of the nematode in the root. Location in the root exterior to the vascular cambium resulted in hyperplasia forming a burrow for the nematode. Root penetration interior to the vascular cambium resulted in giant cells contiguous to the anterior region of the root-knot nematode. The giant cells occupied extensive regions of the root disrupting the xylem and obliterating cells in the penetration pathway. Giant cells were multi-nucleate with dense cytoplasm and thickened cell walls.
I. Yates, D. Sparks, and W.M. Powell
C.A. Powell, W.L. Mountain, and M.A. Derr
W.L. Mountain, C.A. Powell, and L.B. Forer
A trough system was developed to study rates of plant virus transmission by plant parasitic nematodes. Perforated plumber's polyvinyl chloride pipe, 5 cm in diameter, was cut into 48-cm lengths, split longitudinally, and fashioned into troughs to hold soil and common dandelion (Taraxacum officinale Webber) transplants. The first plant in each trough was infected with tomato ringspot virus (TmRSV), followed by 10 uninfected plants spaced at 4-cm intervals. The soil contained a high concentration of Xiphinema rivesi (199 per 100 cm3), a low concentration (16 per 100 cm3), or none. Plants were assayed biweekly for TmRSV. After 42 weeks, transmission rates between the low and high concentrations of nematodes were not significantly different. The subirrigated trough system provided excellent soil conditions for plant growth and sufficient nematode survival to detect virus transmission through 36 weeks.
W. Patrick Wechter, Mark W. Farnham, J. Powell Smith, and Anthony P. Keinath
Brassica leafy greens (Brassica juncea L. and Brassica rapa L.) represent one of the most economically important vegetable crop groups in the southeastern United States. In the last 10 years, numerous occurrences of a leaf-spot disease on these leafy vegetables have been reported in several states. This disease, known as peppery leaf spot, is now causing serious crop losses and has been attributed to the bacterial phytopathogen Pseudomonas syringae pv. maculicola (Psm). To date, it appears that all cultivars of the Brassica leafy greens are susceptible, and available pesticides for control of this disease appear unable to reduce the disease to acceptable levels. Thus, we undertook a search for potential resistance to this disease among accessions of B. juncea and B. rapa included in the U.S. Plant Introduction (PI) collection. In greenhouse trials, we screened commercial cultivars and 672 U.S. PIs (226 B. juncea and 446 B. rapa) for resistance to Psm with artificial inoculation. Although severity of disease symptoms was significantly different among inoculated accessions, no acceptable levels of resistance were found in any of the more than 400 B. rapa accessions tested. Only two B. juncea accessions (PI 195553 and G 30988) of 226 tested had acceptable levels of resistance that might prove economically useful.
J.M. Goatley Jr., A.J. Powell Jr., M. Barrett, and W.W. Witt
Laboratory studies were conducted to determine the basis for chlorsulfuron selectivity between Kentucky bluegrass (Poa pratensis L. cv. Kenblue) and tall fescue (Festuca arundinacea Schreb. cv. Rebel). Tall fescue absorbed and translocated more foliar-applied [14C]-labeled chlorsulfuron from the treated leaf than Kentucky bluegrass. The two species absorbed similar amounts of chlorsulfuron from nutrient solution into the roots, but tall fescue translocated more of the absorbed radioactivity to the shoots. Tall fescue metabolized chlorsulfuron in the shoots slightly more slowly than Kentucky bluegrass. Allof these factors apparently contributed to the higher tolerance of Kentucky bluegrass than of tall fescue to chlorsulfuron. Chemical name used: (2-chloro-N-[[4-methoxy-6-methyl-1,3,5 -triazin-2-yl)amino]-carbonyl] benzenesulfonamide) (chlorsulfuron).
W. Patrick Wechter, Melanie M. McMillan, Anthony P. Keinath, J. Powell Smith, and Mark W. Farnham
J.M. Goatley Jr., A.J. Powell Jr., W.W. Witt, and M. Barrett
Chlorsulfuron, diclofop, and sulfometuron were evaluated for potential use in selective control of tall fescue (Festuca arundinacea Schreb.) in Kentucky bluegrass (Poa pratensis L.). Polynomial trend analyses indicated highly significant linear and quadratic response curves for percentage of tall fescue reduction for each herbicide. Fall and spring treatments with chlorsulfuron and diclofop provided significant tall fescue control, with slight to moderate initial Kentucky bluegrass phytotoxicity. Fall and spring applications of sulfometuron resulted in excellent tall fescue control, but initial Kentucky bluegrass damage was severe and would be unacceptable for high maintenance turf. Chemical names used: 2-chloro- N -[[(4-methoxy-6-methyl-l,3,5-triazin-2-yl)amino]carbonyl]-benzenesulfonamide (chlorsulfuron); 2-[4-(2,4-dichlorophenoxy)phenoxy]proponoate (diclofop); N -[[(4,6-dimethylpyrimidin-2-yl)amino]carbonyl]-2-methoxycarbonyl-benzenesulfonamide (sulfometuron).
Mark W. Farnham, Glen Ruttencutter, J. Powell Smith, and Anthony P. Keinath
Collard (Brassica oleracea L. Acephala Group) is a leafy green vegetable adapted to the southeastern United States. The number of commercially available collard cultivars is limited, and the most popular cultivars are susceptible to fusarium yellows, a disease that most cabbage (B. oleracea Capitata group) cultivars are resistant to. We hypothesized that hybrids of cabbage and collard would look more like collard, because heading of cabbage is at least partially recessive to the nonheading growth habit of collard. We also postulated that cabbage–collard hybrids might be used directly as collard cultivars. To test these postulates, cytoplasmic male sterile cabbage inbreds were crossed to different male fertile collard inbreds and hybrid seed was produced. Resulting cabbage–collard hybrids were compared to conventional collard cultivars in three replicated field trials in South Carolina. In all trials, cabbage–collard hybrids exhibited size and weight more similar to conventional collard than cabbage, and throughout most of the growing season the collards remained nonheading. In addition, the cabbage–collard hybrids were much more uniform than open-pollinated collard cultivars. Among cabbage–collard hybrids there was significant variation with some hybrids appearing more collard-like than others. The collard inbreds designated A and B may have the greatest potential for making promising cabbage–collard hybrids. Particular hybrids (i.e., A3 or B2), derived from these inbreds and tested in this study, can perform better than certain conventional collards and may serve as possible new cultivars of this vegetable crop.
J.M. Vogel, A. Rafalski, M. Morgante, G. Taramino, W. Powell, M. Hanafey, and S.V. Tingey
DNA-based diagnostics are now well-established as a means to assay diversity at the locus, chromosome, and whole-genome levels. As technology has advanced, DNA sequence-based assays have become easier to use, more efficient at screening for nucleotide sequence-based polymorphisms, and available to a wider cross-section of the research community. A review of the use of molecular markers in several different areas of genetics and plant breeding will be presented, as well as a discussion about their advantages and limitations. Recent advances in several areas of technology development and laboratory automation will also be presented, including a summary of direct comparison of different DNA marker systems against a common set of soybean cultivars.