Two isolates of the barley net blotch pathogen (Pyrenophora teres f. teres), one possessing high virulence (0-1) and the other possessing low virulence (15A) on the barley cultivar Harbin, were crossed and the progeny of the mating were isolated. Conidia from cultures of the parent and progeny isolates were used as inoculum to determine the inheritance of virulence in the pathogen. Of the 82 progeny tested, 42 exhibited high virulence and 40 exhibited low virulence on 'Harbin' barley. The data support a model in which a single, major gene controls virulence in P. teres f. teres on this barley cultivar (1:1 ratio; chi(2) = 0.05, P = 0.83). Preparations of DNA were made from parental and progeny isolates, and the DNA was subjected to the random amplified polymorphic DNA (RAPD) technique in a search for molecular genetic markers associated with the virulence phenotype. Five RAPD markers were obtained that were associated in coupling with low virulence. The data indicate that the RAPD technique can be used to tag genetic determinants for virulence in P. teres f. teres.

Isolates of C. finzbriata from different hosts and geographic locations diftered in colony type, pathogenicity, growth rate, and conidial states, but were similar in certain other characters, including an absolute requirement of thiamine for the production of perithecia, opti~num te~nperature for growth, perithecial morphology, and sexual behavior.The amount of cross-fertility between strains from different hosts and geographic locations was determined. The significance of ~norphological differences and cross-fertility between strains is discussed ill relationship t o the taxonomy of C. fit~ibriata and the related species C. nzonilifornzis (Hedge.) C. hloreau. C. variospora Davids. is co~lsidered a synonym of C. fivrbrzata.

Soybean (Glycine max L.) is a major crop grown in the United States but is susceptible to many diseases that cause significant yield losses each year. Consistent threats exist across both northern and southern production regions and include the soybean cyst nematode, charcoal rot, and seedling diseases. In contrast, significant soybean diseases like Phytophthora stem and root rot, sudden death syndrome, and Sclerotinia stem rot (white mold) are intermittent threats that can be heavily influenced by environmental factors. Additional threats to soybean production that have emerged in recent years as more common problems in soybean production include root-knot and reniform nematodes, frogeye leaf spot, and Diaporthe diseases. Disease in any crop will only occur when the three components of the disease triangle are present: a susceptible host, a virulent pathogen, and a conducive environment. If an environment is becoming more conducive for a particular disease, it is important that farmers and practitioners are prepared to manage the problem. The information in this review was compiled to help assist agriculturalists in being proactive in managing new soybean diseases that may be emerging in new areas. To do this, we provide: 1) an overview of the impact and disease cycle for major soybean diseases currently causing significant yield losses in the United States, 2) a comprehensive review of the current management strategies for each soybean disease, and 3) insights into the epidemiology of each pathogen, including the likelihood of outbreaks and expansion to additional geographic regions based on current trends in climate change.

Rice blast, caused by Pyricularia grisea, was first found in California in 1996. Disease surveys have shown the blast disease is spreading at a moderate rate in California rice fields. Although no effective major resistance genes are known to occur in widely grown commercial California cultivars, there appear to be differences among the cultivars with respect to field susceptibility to the pathogen. P. grisea was recovered from rice crop residue and commercial seedlots which are suggested as possible sources of initial P. grisea inoculum in California rice fields. Examination of weather data indicates that environmental conditions in California rice-producing areas are permissive for rice blast but generally not optimal for epidemic development. Spore trapping determined that the majority of P. grisea conidia are generally not released until approximately 6:00 A.M. and would not have sufficient time for infection before leaf wetness periods end. Azoxystrobin showed positive results with respect to reduction of neck blast incidence and yield increases in small-plot and large-scale fungicide trials.

The recent observance of Fusarium fujikuroi, the causal agent of Bakanae disease of rice, in California provides a unique opportunity to assess the population diversity of an introduced pathogen in a new environment. We collected 172 isolates of this pathogen between 2000 and 2003 from California rice and two from water grass (Echinochloa spp.). Pathogenicity of F. fujikuroi was demonstrated on early water grass (E. oryzoides) and barnyard grass (E. crus-galli) indicating that weed control should be part of Bakanae management programs. Both mating types and six unique amplified fragment length polymorphism haplotypes corresponding to six identified vegetative compatibility groups were detected. The two most frequently isolated haplotypes encompassed 94% of the collected isolates, suggesting that clonal reproduction dominates. Coefficients of similarity between the unique haplotypes ranged from 0.94 to 0.98, and indicate that there is very little genotypic variation in the F. fujikuroi population in California. The near fixation of the MAT-1 idiomorph (observed ratio 170 MAT-1:4 MAT-2), is consistent with a hypothesis of predominant or exclusive asexual reproduction. The low level of introduced genotypic diversity, in conjunction with the asexual reproductive strategy of this population will slow evolutionary processes, including adaptation to the California environment.

No abstract

No abstract

Cultivar mixtures have been suggested as a means to control foliar diseases in small grains, but little information is available on how to choose component lines for use in cultivar mixtures. This 3‐yr study tested the ability of two‐, three‐, and four‐component barley (Hordeum vulgare L.) mixtures to control scald disease caused by the fungus Rhynchosporium secalis (Oud) Davis. The mixtures were composed of lines from the parents of Composite Cross II (CCII), a naturally evolving barley population, and from the 45th generation of CCII. The lines differed in resistance or susceptibility to four diagnostic pathotypes of the scald fungus. The results indicate that some barley lines, when grown in mixture, interact to reduce or increase incidence of scald. Lines from the 45th generation of CCII interacted to reduce incidence of scald more often than parental lines of CCII. The largest reductions in sca.ld were observed in mixtures containing lines that were susceptible when grown in pure stand. Genes for resistance to R. secalis were deployed singly in mixtures and also pyramided into single lines. Differences between these methods of gene deployment were nonsignificant statistically. Some two‐component mixtures made up of one resistant and one susceptible line and some three‐component mixtures made up of two susceptible components and one resistant component had no more disease than the resistant component of the mixture grown alone. The results were interpreted as indicating that cultivar mixtures offer an effective strategy for controlling damage due to R. secalis.