Didymella bryoniae (Auersw.) Rehm [anamorph Phoma cucurbitacearum (Fr.) Sacc.], the plant pathogenic fungus that causes gummy stem blight and black rot on cucurbits, was first described in 1869 from Bryonia (bryony or wild hops) in central Europe. Today, this pathogen is found on six continents on at least 12 genera and 23 species of cucurbits. How did D. bryoniae progress from a pathogen of a native plant in central Europe to a worldwide threat to cucurbits cultivated in humid environments? Clues from the early discoveries of this fungus, its characteristics as a seedborne pathogen, and its broad adaptation to cucurbit hosts will provide some answers to this question.
Mark W. Farnham and Anthony P. Keinath
Wirestem, caused by Rhizoctonia solani, is a destructive disease of B. oleracea cole crops and is distributed worldwide. Effective means of wirestem control include soil fumigation and soil treatment with pentachloronitrobenzene, which are increasingly expensive and environmentally undesirable. As a consequence, alternative methods of wirestem control are needed. Thus, we conducted controlled-environment and field experiments to develop methodology to study host-plant resistance and possibly biocontrol agents as potential wirestem control alternatives. Seedlings of 12 cultivars (three each of cauliflower, broccoli, cabbage, and collard) at the four- to five-leaf stage were transplanted to trays in a growth room or into field plots and covered with soil infested with R. solani AG-4 sclerotia. Disease progression (percent of plants healthy, diseased, and dead) was observed every 3 days for 2 weeks in the controlled environments and for 3 weeks in field trials. At the end of two studies, plants were dug with roots intact and rated for disease using a 1 to 10 scale. In all trials, percent healthy plants stabilized at about 2 weeks after inoculation. Incidence of wirestem disease varied among experiments ranging from 70% to 100% diseased, dead plants in controlled environments, and from 51% to 88% and 33% 65% in the two respective field studies. Disease rating was always negatively and significantly correlated with percent healthy plants. Although a genotype × environment interaction was observed, some cultivars (i.e., `Snowcone' and `Snowcrown' cauliflowers) were always severely diseased, while others (i.e., `Viking' broccoli and `Blue Max' collard) were consistently among the least diseased.
Anthony P. Keinath and Virginia B. DuBose
Twenty-six cultivars and two numbered selections of Cucurbita pepo L. pumpkin and four cultivars of C. maxima Duchesne pumpkin were evaluated in field experiments in 1996 and 1997 in Charleston, S.C. The four C. maxima cultivars (`Mammoth Gold', `Big Max', `Rouge Vif d'Etamps', and `Lumina') and three C. pepo cultigens (HMX 6686, HMX 6688, and Magic Lantern) had lower powdery mildew [Sphaerotheca fuliginea (Schlechtend.:Fr.) Pollacci] severities than did the other C. pepo cultivars. Overall, C. maxima cultivars also had less foliage showing virus symptoms and less downy mildew [Pseudoperonospora cubensis (Berk.& M.A. Curtis) Rostovzev] than did C. pepo cultigens. Mid- and long-season cultigens of both species (≥100 days to maturity) produced a greater number of marketable-quality fruit than did short-season cultigens. Cucurbita maxima and C. pepo produced similar numbers of marketable fruit; however, more potential marketable yield was possible in C. maxima since most fruit were affected by virus. The C. pepo cultigens Spookie, HMX 6686, and Spooktacular produced the greatest numbers of marketable fruit. In general, no cultigens were well-adapted to the growing conditions of the humid coastal plain of the southeastern United States.
James W. Rushing, Anthony P. Keinath, and Wilton P. Cook
Watermelon fruit blotch (WFB) symptoms did not appear on healthy watermelon [Citrullus lanatus (Thunb.) Matsum. and Nakai] fruit placed in contact with the diseased surface of other fruit and stored at either 52 or 68 °F (11 or 20 °C) for 1 week. After 3 weeks in storage, some WFB transmission was observed and the frequency of transmission was greater at 68 than 52 °F. Surface abrasion of either the healthy fruit, diseased fruit, or both fruit did not promote transmission of WFB compared with unabraded controls. Some healthy fruit harvested from a field with diseased fruit developed very minor symptoms of WFB in postharvest storage, but the symptoms were not severe enough to cause market problems. Harvesting appeared to halt the spread of WFB symptoms on individual fruit with less than ≈10% of the fruit surface affected at harvest. If care is taken during harvest and grading to exclude diseased fruit, and if proper precooling and subsequent temperature management is implemented for marketable fruit, WFB does not appear to be of concern for the marketing of watermelons.
James W. Rushing, Wilton P. Cook, and Anthony P. Keinath
Transmission of watermelon fruit blotch was not observed during the first week of postharvest storage at either 11 °C or 20 °C when the diseased surface of an affected watermelon was placed in direct contact with the surface of a healthy fruit. Abrasion of either the diseased fruit, the healthy fruit, or both fruit did not facilitate transmission of the disease compared to unabraded controls. After 3 weeks' storage, disease transmission was significantly greater at 20 °C than at 11 °C, illustrating the importance of appropriate temperature management during postharvest handling. Harvesting appears to arrest the development of watermelon fruit blotch if the disease symptoms cover less than approximately 5% of the fruit surface at harvest.
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.
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.
Howard F. Harrison, D. Michael Jackson, Anthony P. Keinath, Paul C. Marino, and Thomas Pullaro
Fall transplanted `Commander' broccoli (Brassica oleracea Botrytis group) yield in mulches formed from the residues of killed cowpea (Vigna unquiculata), soybean (Glycine max), and velvetbean (Mucuna pruriens) cover crops was compared to yield in conventional production on bare soil. Average aboveground biomass production was 6.9, 7.7, and 5.9 t·ha-1 (3.08, 3.43, and 2.63 tons/acre) and total nitrogen content of the aboveground tissues was 2.9%, 2.8%, and 2.7% of the dry weight for cowpea, soybean, and velvetbean, respectively. Within each cover crop mulch main plot, subplots received different nitrogen rates, [0, 84.1, or 168.1 kg·ha-1 (0, 75, or 150 lb/acre)]. For several nitrogen level × year comparisons, broccoli grown in mulched plots yielded higher than broccoli grown on bare soil plots. Cowpea and soybean mulches promoted broccoli growth more than velvetbean mulch. The mulches of all three species persisted through the growing season and suppressed annual weeds.
Amnon Levi, Claude E. Thomas, Anthony P. Keinath, and Todd C. Wehner
Genetic relatedness was estimated among 42 U.S. plant introduction (PI) accessions of the genus Citrullus (37 PIs of which were reported to have disease resistance and five watermelon cultivars) using 30 RAPD primers. These primers produced 662 RAPD markers that could be scored with high confidence. Based on these markers, genetic similarity coefficients were calculated, and a dendrogram was constructed using the unweighted pair-group method with arithmatic average (UPGMA). The analysis delineated three major clusters. The first cluster consisted of a group of five watermelon cultivars, a group of C. lanatus var. lanatus accessions and a group of C. lanatus var. lanatus accessions that contained some C. lanatus var. citroides genes. The second cluster consisted of the C. lanatus var. citroides accessions, while the third cluster consisted of the C. colocynthis accessions. The two C. lanatus clusters differentiated from each other and from the C. colocynthis cluster at the level of 58.8% and 38.9% genetic similarity. Our results indicate that closely related Citrullus PIs may have resistances to the same diseases. Thus, molecular markers may be a useful tool in the development of core collections of Citrullus PIs with resistance to diseases.