The ultrastructure of an aster yellows mycoplasma-like organism was studied in the phloem of periwinkle (Vinca rosea L.) plants. Banded filaments were observed in association with mycoplasma-like organisms of characteristic morphology. The filaments were variable in length, from 50-100 nm in width, and displayed a regular periodic banding of alternating electron-dense and electron-lucent structures.

Baseline information on the diversity and population densities of fungi collected from soil debris and cotton (Gossypium hirsutum L.) roots was determined. Samples were collected from Tifton, GA, and Starkville, MS containing cotton field soil treated with the nematicides 1,3-dichloroproprene (fumigant) and aldicarb (granules). A total of 10,550 and 13,450 fungal isolates were collected from these two study sites, respectively. Of this total, 34 genera of plant pathogenic or saprophytic species were identified. Pathogenic root fungi included Fusarium spp. (40% of all isolations), Macrophomina, Pythium, Rhizoctonia, and Sclerotium. Fusarium and Rhizoctonia were the most common fungal species identified and included F. oxysporum, F. verticillioides and F. solani, the three Fusarium species pathogenic on cotton plants. Population densities of Fusarium were not significantly different among locations or tissue types sampled. Macrophomina was isolated at greater numbers near the end of the growing seasons. Anastomosis groups of R. solani isolated from roots and soil debris included AG-3, -4, -7, 2-2, and -13 and anastomosis groups of binucleate Rhizoctonia included CAG-2, -3, and -5. Occurrences and frequency of isolations among sampling dates were not consistent. Fluctuations in the frequency of isolation of Rhizoctonia did not correspond with changes in frequency of isolation of the biological control fungus, Trichoderma. When individual or pooled frequencies of the mycobiota were compared to nematicide treatments, no specific trends occurred between treatments, application methods or rates. Results from this study show that use of 1,3-D and aldicarb in cotton fields does not significantly impact plant pathogenic fungi or saprophytic fungal populations. Thus cotton producers need not adjust seedling disease control measures when these two nematicides are used.

Tuber initiation and development are processes basic to potato production and are particularly critical in areas with short growing seasons. It is important to know how and to what extent management decisions affect these processes in order to maximize the yield of marketable tubers. A two-year field study, conducted in southcentral Alaska, examined top growth, tuber initiation, and tuber development in eight potato cultivars grown with and without irrigation. Plants of the cultivars Allagash Russet, Bake-King, Green Mountain, Kennebec, Lemhi Russet, Russet Burbank, Shepody, and Superior were harvested weekly throughout the growing season, and top dry weight, numbers of tubers, and individual tuber fresh weights were recorded. Top dry weight was reduced by moisture stress shortly after emergence in 1993, and about one month following emergence in 1994, when early-season soil moisture was greater. The weight of tubers was similarly affected within approximately 5 wk of emergence in 1993 and 6 weeks in 1994. Tuber weight at harvest was increased two-to three-fold by irrigation in all cultivars. The number of tubers each plant set was affected by irrigation in most, but not all, cultivars. Some varieties (Lemhi Russet in 1994, Allagash Russet both years) set more tubers than were maintained through the growing season. Tuber remnants found during sample collection indicated that tuber reabsorption had occurred. Irrigated Green Mountain had more than one tuber initiation period during the season, whereas other varieties such as Shepody maintained a relatively constant number of tubers following initial tuber set. Tuber size distribution at the end of the growing season showed that larger tubers were favored by irrigation.

Web (aerial) blight was observed in field plots of Catharanthus roseus (L.) G. Don (Madagascar periwinkle) during three consecutive summers at the Burden Research Center in Baton Rouge. Leaf spots formed first, followed by a general blighting of leaves and stems that resulted in circular areas of dead plants in the plots. Dead leaves were matted together but remained attached to plants. Mycelia, and occasionally small, brown sclerotia (1 to 3 mm) were observed on blighted foliage. During the first year, only prostrate-growing cultivars belonging to the Mediterranean series of C. roseus were infected, but in 2001 and 2002 upright-growing cultivars as well as those with prostrate growth habit became infected. The disease occurred in July and August during periods of hot, humid, and rainy weather. Among 52 cultivars in the 2001 trial, only ‘Tropicana Pink’, ‘Tropicana Rose’ and ‘Stardust Orchid’ were disease free. A Rhizoctonia sp. was consistently isolated from diseased plants and further characterized as R. solani Kühn AG-1 based on its multinucleate cells and hyphal anastomosis with several AG-1 tester isolates. On potato dextrose agar, colonies displayed morphologies with characteristics of AG-1 IA and AG-1 IB, therefore, identification to AG subgroup was not made. Mature colonies ranged from light tan to brown and produced sclerotia, individually or in clumps, at the edge of the culture dish. Pathogenicity tests were performed by placing agar blocks, taken from the margins of 7-day-old cultures, on stems of eight healthy Madagascar periwinkle plants (15 to 20 cm tall). Inoculated and noninoculated control plants were held in a dew chamber at 26°C for 3 days and then moved to a greenhouse. Leaves on all inoculated plants developed water-soaked spots that turned dark brown or black prior to death, whereas noninoculated plants remained healthy. R. solani was reisolated from inoculated plants and its cultural characteristics were similar to those of the original isolate. Web blight occurs in Louisiana on Madagascar periwinkle used as landscape bedding plants, but has not been observed on container-grown plants. Web blight caused by R. solani AG-1 was previously reported on Madagascar periwinkle from Alabama (1). R. solani AG-1 has been reported previously as causing web blight in Louisiana on rosemary (2), dianthus (4), and verbena (3). To our knowledge, this is the first report of web blight on Madagascar periwinkle (C. roseus) in Louisiana. References: (1) A. K. Hagan and J. M. Mullen. Plant Dis. 77:1169, 1993. (2) G. E. Holcomb. Plant Dis. 76:859, 1992. (3) G. E. Holcomb and D. E. Carling. Plant Dis. 84:492, 2000. (4) G. E. Holcomb and D. E. Carling. Plant Dis. 84:1344, 2000.

Anastomosis group (AG) identity of Rhizoctonia solani isolates collected from potato plants growing near Leon, Guanajuato, in central Mexico was determined. In samples from 15 fields, we found AG-3 and AG-4 with a frequency of 73.5% and 26.5% respectively. AG-4 was found only during the flowering stage of plant growth, whereas AG-3 was present on plants at every stage of development. The efficacy against R. solani, in vitro and in situ , of certain biocontrol agents ( Bacillus subtilis, Gliocladium virens ) and fungicides (Pencycuron, Tolcoflos-methyl, Fluazinam, Azoxystrobin) was assessed. Certain Bacillus strains collected from the field inhibited in vitro growth of isolates of certain AGs. Only Pencycuron and Tolcoflos-methyl inhibited AG-3 100% in vitro , whereas in the field, Azoxystrobin and Pencycuron showed the highest efficacy against development of sclerotia on tubers.

Dianthus chinensis (rainbow pink) is a popular seasonal bedding plant for the Gulf Coast of the United States and is primarily grown during the fall, winter, and early spring months. In August 1999, diseased plants were observed in a Baton Rouge, LA, propagation nursery with irregularly oval, tan leaf spots 3 to 10 mm in diameter. Heavily infected leaves became blighted and were killed, but plants survived and roots, crowns, and flowers were not affected. Infected leaf samples were surface-disinfected for 1 to 3 min in 70% ethyl alcohol, blotted dry, and sections were placed on 2% acidified water agar. A fungus that was identified as Rhizoctonia solani, and belonging to anastomosis group (AG)-1 IB, was consistently isolated from infected leaves. Inoculum was prepared by blending one 7-day-old plate culture, grown on acidified potato-dextrose agar, in 100 ml distilled deionized water. Pathogenicity tests were performed by dripping inoculum from a 10-ml pipette on leaf surfaces of healthy rainbow pink plants. Inoculated and noninoculated plants were held in a dew chamber at 26°C for 2 to 3 days and then removed to a greenhouse where temperatures ranged from 25 to 32°C. Inoculated plants developed water-soaked spots after 2 to 3 days that turned tan and became necrotic 5 to 10 days later. These symptoms were like those observed on the original diseased plants. R. solani was reisolated from inoculated plants, and noninoculated plants remained healthy. Although R. solani has been reported previously as a root and stem pathogen of D. chinensis (1), this is the first report of leaf blight disease caused by this fungus. Reference: (1) D. F. Farr et al. 1989. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN.

ABSTRACT Crater disease (CD) of wheat is caused by a Rhizoctonia solani strain of ambiguous phylogeny. Anastomosis reactions confirmed placement of CD-causing R. solani in anastomosis group (AG) 6, with results indicating a closer affinity to AG-6 GV than to AG-6 HG. Cultures of CD isolates were initially white to cream, turning a yellowish light brown after 10 days. Concentric rings of dark and light mycelium were evident from an early stage. Mycelium generally was appressed to the agar surface, with sparse aerial growth. A few light-colored, irregularly shaped sclerotia could be discerned after 2 weeks. The mean hyphal diameter of CD-causing R. solani was 7.46 mum (ranging from 5.0 to 10.0 mum), and cells contained a mean number of four (ranging from two to eight) nuclei, compared to a mean hyphal diameter of 8.58 and 8.42 mum and a mean nuclear number of six and four for AG-6 HG and AG-6 GV, respectively. The CD isolates had a slower growth rate (15.3 mm/day) than AG-6 HG (29.1 mm/day) and AG-6 GV (22.6 mm/day) but, like AG-6, were thiamine prototrophic. Conspicuous nodulose swellings were produced by CD-causing R. solani on roots of wheat, and infection resulted in retarded shoot growth. Smaller nodules were evident on bean and soybean roots. Fingerprint patterns generated for the various isolates with four enzymes, HpaII, Sau3AI, TaqI, and CfoI, showed the presence of a unique 610-bp fragment in the pathogen. It is proposed that CD-causing R. solani isolates represent a distinct intersterility group within AG-6 that is more related to subgroup GV than to subgroup HG.

An examination of microscopic and macroscopic vegetative reactions between paired isolated of Rhizoctonia solani AG-8 revealed that pairs of isolates that give a C2 anastomosis reaction at the microscopic level always gives a ‘tuft’ reaction at the macroscopic level. Similarly, any pair that gives a C3 anastomosis reaction at the microscopic level always gives a ‘merge’ reaction at the macroscopic level. Thus, macroscopic vegetative (‘tuft type’) reactions can be used to predict the microscopic (anastomosis) reaction (and vice versa) between paired isolate of AG-8. This pattern also seems to apply to paired isolates of AG-3, AG-7 and AG-10 but not the other AGs of R. solani . The C2 anastomosis reaction is always observed when two isolates from different zymogram groups of R. solani AG-8 are paired. When isolates from within any of the five zymogram groups comprising AG-8 are paired, either a C2 or a C3 anastomosis reaction will result. Many field isolates of AG-8, when paired, give a C3 anastomosis reaction. We propose that a group of field isolates of R. solani that give a C3 anastomosis reaction be designated a ‘vegetatively compatible population’ (VCP).

Crater disease (CD) of wheat (Triticum aestivum L.) on the Springbok Flats, South Africa, and bare patch disease of cereals occurring elsewhere in the world, are both caused by Rhizoctonia solani Kühn (Samuel & Garrett, 1932; Deacon & Scott, 1985), albeit by different anastomosis groups of the pathogen. Bare patch is ascribed to root infection by anastomosis group (AG) 8 (Neate & Warcup, 1985), whereas recent evidence (Carling et al., 1996) assigned the CD R. solani to AG-6. However, unlike other isolates of AG-6 (and all other AGs of R. solani for that matter), the CD AG-6 also produces bridging reactions with R. solani AG-1 IA (Carling et al., 1996). Furthermore, AG-6 has previously been found only in non-pathogenic association with orchid roots (Uetake et al., 1988) turfgrass (Zhang & Dernoeden, 1995) and as inhabitant of uncultivated soil (Kuninaga et al., 1979), whereas the CD R. solani causes a destructive disease. Considering the polymorphic and phylogenetically-diverse nature of AG-6 (Kuninaga & Yokosawa, 1984a) it seems reasonable to assume that the CD R. solani could constitute a separate subgroup within this AG.

The individual and combined effects of two arbuscular mycorrhizal fungi (AMF), Meloidogyne arenaria, and phosphorus (P) fertilization, (0, 25, 75, and 125 μg/g soil) on peanut plant growth and pod yield were determined in greenhouse studies. Best growth and yield usually occurred at 75 or 125 μg P regardless of inoculation treatment. Peanut growth and yield were generally stimulated by AMF development, and growth alone was suppressed by M. arenaria at 0 and 25 μg P. In challenge inoculations, VAM increased peanut plant tolerance to the nematode and offset the growth reductions caused by M. arenaria at the two lower P levels. However, VAM and added P increased galling and M. arenaria egg production/g root, thereby increasing peanut susceptibility to nematode attack. M. arenaria had only a minimal effect on root colonization by AMF and sporulation by the fungi.

Isolates of Rhizoctonia solani AG-10 have been collected at low frequency from wheat and barley roots in Washington and Oregon. In Western Australia, AG-10 is collected regularly, but at low frequency, from the roots of cereal, lupin and pasture species. In this study all attempts to induce AG-10 to sporulate were unsuccessful. However, earlier studies by one author (A.O.) confirmed that the teleomorph of AG-10 conforms with that of Thanatephorus cucumeris. On potato-dextrose agar, mycelial growth of AG-10 is white to light tan with some isolates displaying a yellowish colouration as the culture ages. White, concentric growth rings form after 3 or 4 days and the formation of embedded gold, orange-gold or very light tan sclerotia occurs between 14 and 20 days. Sclerotia vary in size from small to large (approximately 2 mm to 8 mm) and are generally irregular in shape. AG-10 is auxotrophic for thiamine. AG-10 is non-pathogenic on wheat, barley, lupin, vetch and tomato, but may be a weak pathogen on cruciferous hosts.

Soil transfers from an intermediate successional site and a mature forest site were applied to Populus balsamifera L. cuttings and Alnus crispa (Ait.) Pursh seedlings placed on an abandoned mined site in south central Alaska to improve plant establishment. Mycorrhizal fungi in the soil transfers from the two successional stages were hypothesized to have different effects on plant species that colonize disturbed sites at different times or on different substrates. The site consisted of coarse, dry, low-nutrient spoils and was naturally colonized by scattered P. balsamifera but not A. crispa, although seed sources for both were adjacent to the site. Physical dimensions of the transplanted seedlings and cuttings were measured at the beginning and end of each growing season. Selected plants were harvested at the end of the 2-year study and examined for mycorrhizal formation, current growth, and leaf tissue nutrient concentrations. Both plant species were taller when treated with the soil transfers from the mature forest than with soils from the intermediate site although the increase for A. crispa was greater. Physical dimensions, current growth, and nutrient concentrations were greater when A. crispa was treated with the mature soil transfer compared with the intermediate soil transfer. Mycorrhizae which infected Alnus were predominantly a brown woody type, while other types accounted for greater relative mycorrhizal infection percentage on Populus. Insufficient quantities of mycorrhizal inoculum of suitable species, as well as low moisture and low nutrient conditions, may be factors limiting A. crispa colonization on primary disturbed sites in south central Alaska.

Head lettuce (Lactuca sativa L.) cv. Salinas was produced in field trials in southcentral Alaska with varying planting dates, planting methods, N sources, and N application rates. Variables measured included head weight and diameter and harvest date. Nitrogen source had little effect on head weight. Direct-seeded lettuce produced heaviest beads from early plantings; transplants produced heaviest heads when planted in mid- to late season. Transplanting generally produced heavier heads than direct-seeding. Head weight of transplanted and direct-seeded lettuce was maximized with ≈112 kg N/ha. The data suggest that 112 kg N/ha may be suitable for lettuce direct-seeded or transplanted throughout the growing season.