J. W. Hoy

Species-specific polymorphisms in transcribed ribosomal DNA of fivePythium species

Abstract Twenty-five isolates representing fivePythium species collected from diverse hosts and geographic origins were evaluated using polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) analysis. DNA regions coding for the small-subunit ribosomal RNA (SrDNA) and the internal transcribed spacer (ITS) were amplified and analyzed by restriction enzyme digestion. The amplified SrDNA was about 1800 bp long and uniform in length among the five species. However, restriction digestion revealed three polymorphic groups. They areP. arrhenomanes andP. graminicola,P. irregulare andP. spinosum, andP. ultimum. The amplified-ITS region showed three different lengths which corresponded to the three polymorphic groups of SrDNA. Each length variant of the ITS showed distinct banding patterns after restriction enzyme digestion. In addition,P. irregulare andP. spinosum each showed distinct banding patterns after digestion with enzymesHinfI andMboI. Physical maps of the restriction sites in the SrDNA and the ITS were determined. Length variation occurred primarily in the spacer between the SrDNA and 5.8 S rDNA; although, it also was detected in the ITS-2 region. Little intraspecific variation was observed in the SrDNA and ITS, and species could be reliably distinguished by RFLP analysis of the amplified rDNA regions. Data presented do not support the maintenance ofP. arrhenomanes andP. graminicola as distinct species. Results indicate that PCR-RFLP can be used as a simple and speedy taxonomical tool for ecological studies ofPythium species.

Effect of tissue culture explant source on sugarcane yield components

Clonal propagation of sugarcane(interspecific hybrids of Saccharum)is conducive to spread of systemicdiseases, such as ratoon stunting disease,caused by Leifsonia xyli subsp. xyli. This important disease iscontrolled by obtaining and plantinghealthy seed-cane. In Louisiana, commercialseed-cane initially produced through tissueculture is available to sugarcane farmersand is being widely planted. Long-termacceptability of this seed-cane productionmethod depends on the production of healthyplants that do not differ significantly inphenotypic and yield characteristics fromthe clones originally selected and releasedas commercial cultivars. To determinewhether tissue culture affects yield or itscomponents, three cultivars, CP 70-321, LCP85-384, and HoCP 85-845, were compared inthree successive crops initially plantedwith stalks from three sources: plantsderived from callus culture of the leafroll above the apical meristem, directregeneration from the apical meristem, andconventional bud propagation. Stalks ofplants derived from both explant sourceswere typical of seed-cane farmers wouldpurchase for planting that had beenpreviously rogued for phenotypic variantsand increased by bud propagation.Differences in yield components amongtissue culture explant sources and budpropagated cane only occurred in CP 70-321.Stalk diameter and stalk weight were lowerand stalk population was higher for plantsderived from leaf roll callus compared tobud propagated cane. Yield components weresimilar for plants derived from an apicalmeristem and bud propagation. Individualplant phenotypic variants resulting fromsomaclonal variation were not observed inany of the cultivars derived from eitherexplant source. In summary, genotype andexplant source affected persistent, uniformphenotypic variation resulting from tissueculture that changed some yield components. However, apical meristem culture wassuitable for production of seed-cane, assugarcane derived by meristem culture ofthree cultivars did not differsignificantly from the original germplasmfor any measured yield trait.

Organic Material Soil Amendment Effects on Root Rot and Sugarcane Growth and Characterization of the Materials

Soil amendment with different organic materials was evaluated in greenhouse experiments for effects on root rot and growth of sugarcane. Materials included composts prepared from cotton gin trash, cottonwood bark, mixed hardwood bark, municipal solid waste, and municipal yard waste; municipal biosolids; and a sugar mill by-product, filterpress cake. Field soil, steam-treated field soil, and steam-treated soil infested with Pythium arrhenomanes were amended with nonsterile or steam-treated organic materials. A metalaxyl fungicide treatment was included for comparison. When added in nonsterile form, cotton gin trash compost, filterpress cake, and biosolids suppressed disease and increased plant growth in field soil and soil infested with P. arrhenomanes, but this ability was reduced after steam treatment. Bark composts were capable of suppressing root rot and increasing plant growth in field soil and Pythium-infested soil when added in either nonsterile or steam-treated forms. Plant growth in steam-treated soil was not promoted by nonsterile or steam-treated materials. Disease suppression provided by organic materials resulted in plant growth increases generally lower than those resulting from metalaxyl treatment in steam-treated soil infested with P. arrhenomanes, but some amendments resulted in growth increases comparable to those obtained with the metalaxyl treatment in field soil. Municipal waste composts had no effect or were detrimental to sugarcane growth. Differences in microbial community composition and chemical properties, including N content, C:N ratio, and other mineral nutrient levels, distinguished organic materials that may suppress disease and promote plant growth by different mechanisms. Microbial activity level of a material was an indicator of potential for disease suppression. The study results suggest that the severity of root rot in sugarcane may be reduced by amending soil with some organic materials.

Molecular and morphological comparison of Pythium arrhenomanes and P. graminicola

Worldwide collections of Pythium arrhenomanes and P. graminicola were studied morphologically and by examining variation in PCR-amplified rDNAs. Restriction patterns of the internal transcribed spacer and portions of the nuclear large-subunit rDNA were diagnostic for the two species. Isolates of P. graminicola shared identical restriction patterns. Most isolates of P. arrhenomanes exhibited identical restriction patterns; however, two genetic subpopulations, each with multiple isolates from a single geographic location, were detected. A phylogenetic analysis of the molecular data, which included two additional species, indicated that P. aphanidermatum was an outgroup and P. myriotylum was more closely related to P. arrhenomanes than was P. graminicola . No restriction site variation was detected among the four species in the nuclear small-subunit rDNA. Isolates of P. arrhenomanes , including both subgroups, differed from the other three species by one restriction site in the mitochondrial large-subunit rDNA. A mean of less than five antheridia per oogonium characterized isolates of P. graminicola , whereas five or more per oogonium characterized most isolates of P. arrhenomanes . The ranges of isolate means for oogonium/oospore diameter overlapped between the two species. Discriminant function analysis suggested that the number of antheridia per oogonium is more important than oogonium/oospore diameter in separating the two species. The rDNA results indicate that P. arrhenomanes and P. graminicola are distinct species. However, due to similar morphology and intraspecific variation, misidentifications of isolates of the two species have occurred.

Effect of brown rust on yield of sugarcane in Louisiana.

Brown rust, caused by Puccinia melanocephala, can cause severe epidemics in susceptible sugarcane cultivars during spring and early summer in Louisiana. The effect of the disease on yield was evaluated in field experiments conducted during three growing seasons. A mixture of three fungicides-azoxystrobin, propiconazole, and tebuconazole-applied biweekly during the spring epidemic period kept brown rust severity low (<5%), and plants protected by fungicide applications throughout the epidemic provided an estimate of attainable yield for comparison with plants naturally infected with rust. A combined analysis over three seasons estimated brown rust caused reductions of 16 and 14% in cane tonnage and total amount of sucrose produced, respectively, in cv. LCP 85-384. The greatest reduction in total sucrose yield of 22% resulted from the epidemic of longest duration, and stalk weight was negatively correlated with rust severity. Comparisons of the yields obtained from plots in which brown rust was controlled early versus late in the epidemic suggested that the impact of the disease is greatest from the middle to late epidemic period when stem elongation has begun.

Herbicide Effects on Sugarcane Growth, Pythium Root Rot, and Pythium arrhenomanes

ABSTRACT Six herbicides were evaluated for their effects on Pythium root rot and growth of sugarcane in greenhouse experiments and on in vitro mycelial growth rate of Pythium arrhenomanes. Pendimethalin and atrazine were most inhibitory to mycelial growth, but neither reduced root rot severity. Asulam, atrazine, and metribuzin were not phytotoxic to sugarcane and did not affect root rot symptom severity in clay loam or silt loam field soils. Atrazine and metribuzin increased shoot number, and atrazine increased total shoot weight for treated plants in silt loam soil. Glyphosate, pendimethalin, and terbacil were phytotoxic to sugarcane. These herbicides increased root rot severity, but the extent to which growth reductions resulted from increased disease severity or from direct herbicide injury was not clear. Adverse effects on plant growth and root rot severity were greater in clay loam than in silt loam soil. The results suggest that sugarcane injury from some herbicides is compounded by increased severity of root rot.

Comparisons of soluble proteins and isozymes for seven Pythium species and applications of the biochemical data to Pythium systematics.

Seven homothallic Pythium species were compared using SDS-polyacrylamide gel electrophoresis of soluble proteins and starch gel isozyme analysis. Similarities in soluble protein banding patterns and isozyme phenotypes were detected for morphologically distinct species. However, morphologically similar species ( P. aphanidermatum vs. P. deliense and P. arrhenomanes vs. P. graminicola ) were not always distinguishable. Cluster and principle component analysis of data from both soluble proteins and isozymes generally grouped the isolates by species. Levels of intraspecific variation differed between species with each technique. Implications of the results from protein electrophoresis and isozyme analysis for Pythium systematics are discussed.

Incidence of Sugarcane Smut in Louisiana and Its Effect on Yield

hectare. All cane stalks were cut by hand Hoy, J. W., Hollier, C. A., Fontenot, D. B., and Grelen, L. B. 1986. Incidence of sugarcane smut in at ground level, topped above the last Louisiana and its effect on yield. Plant Disease 70:59-60. hard stalk internode, and stripped of all leaf and leaf sheath material. Sugarcane (interspecific hybrids of Saccharum) infected by Ustilago scitaminea was observed in 13 The mean weight for smutfree stalks of 15 sugarcane-growing parishes in 1984. Frequency of observed infested fields and incidence of was estimated for the entire field in two diseased plants within fields were highest in susceptible cultivars CP 73-351 and NCo 310, wastimate for th entireieint intermediate in moderately susceptible cultivars CP 65-357 and CP 74-383, lowest in resistant plt-cantae fields(A and B)by determining cultivars CP 70-330 and CP 72-370, and absent in CP 70-321 and CP 72-356. Cane tonnage yields the mean stalk weight from five 15-stalk were negatively correlated (P= 0.01) with increasing levels of smut incidence observed in small samples collected at different locations plots in three plant-cane and one ratoon-cane field of CP 73-35 . The most significant effect of smut within the field. In the third plant-cane on yield was a reduction in number of healthy stalks in diseased plots, field (C) and the ratoon-cane field (D), a mean weight for smutfree stalks was determined for each individual plot from Smut, caused by Ustilago scitaminea within four commercial fields of CP 73- the total weight of the first 15 smutfree Syd., is an important disease of sugarcane 351 in which smut had spread naturally. stalks in each plot. (interspecific hybrids of Saccharum) that Stalk samples collected from individual has spread to all of the major cane- MATERIALS AND METHODS plots in fields C and D were milled, and growing areas of the world except Survey. A survey was conducted on 61 percent sucrose and fiber content

Sugarcane Leaf Scald Distribution, Symptomatology, and Effect on Yield in Louisiana

Sugarcane leaf scald, caused by Xanthomonas albilineans, was first observed in Louisiana in November 1992. During 1993, surveys were conducted to determine the geographic distribution of the disease and which cultivars were affected. Symptomatology was monitored in fields with diseased plants, and the effect on yield was studied. Leaf scald was detected in commercial fields of five of seven commercial cultivars. The number of fields of cultivars CP 70-321, CP 72-370, and LHo 83-153 affected and the incidence of plants within fields were low. A greater number of affected fields and higher disease incidence within fields were observed for cultivars CP 74-383 and LCP 82-89

Temporal Increase and Spatial Distribution of Sugarcane Yellow Leaf and Infestations of the Aphid Vector, Melanaphis sacchari

Yellow leaf, caused by Sugarcane yellow leaf virus (ScYLV), is a potentially important disease of sugarcane first found in Louisiana during 1996. A survey during 2002 determined that ScYLV infection was present in all sugarcane-production areas of Louisiana. Virus was detected in 48% of 42 fields, and incidence averaged 15% in these fields. Disease progress curves determined in four fields during two growing seasons indicated that the greatest temporal increase of virus infection occurred during late spring and early summer and coincided with the initial infestation and increase of the virus vector, the sugarcane aphid (Melanaphis sacchari). Aphid infestations in the experimental fields during 2002 and 2003 ranged from 1.2 to 33.0 and 1.0 to 4.2 aphids per leaf, respectively. Final disease incidences of 2.9, 5.2, and 5.2% were recorded in three fields planted with virus-free seed-cane. Distribution of ScYLV infections and aphids evaluated with spatial autocorrelation analysis indicated that ScYLV and its aphid vector both exhibited a predominantly random spatial distribution, with occasional aggregation. The low incidence and rates of disease increase observed, despite the widespread occurrence of potential vectors, suggest that inoculum pressure remains low in Louisiana. Therefore, it may be possible to keep yellow leaf at low levels by planting virus-free seed-cane.