Thomas R. Gordon

The Pitch Canker Epidemic in California

Native Monterey pine (Pinus radiata D. Don) forests are currently found at three disjunct locations in coastal California and on two islands off the coast of Mexico. The mainland forests comprise, in total, approximately 4,500 ha (11), whereas the two island populations are limited to 150 ha on Cedros Island and fewer than 400 trees on Guadalupe Island (28). Collectively, these populations are significant ecological and recreational resources, but they are also a valuable repository of useful genetic traits for improved varieties of Monterey pine, which are widely used by the timber industry. In California, Monterey pine is important as a landscape tree, with an estimated 50 million standing trees as of 1985. Monterey pines were especially popular for plantings on freeway rights-of-way, where they served as visual and sound barriers for the adjacent properties. It was among such trees that a dieback problem became apparent in Santa Cruz County (Fig. 1) during the mid-1980s. In 1986, A. H. McCain, extension plant pathologist at University of California Berkeley, established that the affected trees were suffering from pitch canker (30), caused by Fusarium circinatum Nirenberg & O’Donnell (=Fusarium subglutinans (Wollenweb. & Reinking) Nelson, Toussoun, & Marasas f. sp. pini). Subsequent surveys showed the disease to be widespread in coastal Santa Cruz County, with a clearly disjunct infestation being found farther inland, in Alameda County (Fig. 1). Curiously, the disease was also found in Christmas tree farms in Los Angeles and San Diego counties, more than 450 km to the southeast (8).

Pitch canker caused by Fusarium circinatum – a growing threat to pine plantations and forests worldwide

Pitch canker, caused by the fungus Fusarium circinatum, is one of the most important pathogens of Pinus species. Sporadic outbreaks and epidemics caused by this fungus have been reported from numerous countries. Symptoms differ depending on the host species, geographical region, climatic conditions and associated insects. Pitch canker represents a significant threat to countries where non-native and susceptible Pinus spp. are grown intensively in plantations.Athorough understanding of the ecology and epidemiology of the causal agent is an important prerequisite to managing this threat. The aim of this review is to summarise contemporary knowledge relating to the pitch canker pathogen, with a particular focus on its threat to plantation forestry.

One fungus, one name

In this letter, we advocate recognizing the genus Fusarium as the sole name for a group that includes virtually all Fusarium species of importance in plant pathology, mycotoxicology, medicine, and basic research. This phylogenetically guided circumscription will free scientists from any obligation to use other genus names, including teleomorphs, for species nested within this clade, and preserve the application of the name Fusarium in the way it has been used for almost a century. Due to recent changes in the International Code of Nomenclature for algae, fungi, and plants, this is an urgent matter that requires community attention. The alternative is to break the longstanding concept of Fusarium into nine or more genera, and remove important taxa such as those in the F. solani species complex from the genus, a move we believe is unnecessary. Here we present taxonomic and nomenclatural proposals that will preserve established research connections and facilitate communication within and between research communities, and at the same time support strong scientific principles and good taxonomic practice.

Verticillium wilt of cauliflower in California.

Since 1990, commercial cauliflower in coastal California has been severely affected by a vascular wilt disease. Symptoms consist of chlorosis, defoliation, stunting, wilting, and vascular discoloration. Disease has been widespread and has caused significant damage in summer and fall crops. Verticillium dahliae was consistently isolated from xylem tissue in stems and roots of affected plants. Techniques tested for inoculation of cauliflower plants were dipping clipped or nonclipped roots into spore suspensions, injecting spore suspensions into cauliflower stems, and planting seedlings into soil along with an agar block colonized with microsclerotia. Only dipping roots into spore suspensions was consistently successful in causing Verticillium wilt

One fungus, one name: defining the genus Fusarium in a scientifically robust way that preserves longstanding use.

In this letter, we advocate recognizing the genus Fusarium as the sole name for a group that includes virtually all Fusarium species of importance in plant pathology, mycotoxicology, medicine, and basic research. This phylogenetically guided circumscription will free scientists from any obligation to use other genus names, including teleomorphs, for species nested within this clade, and preserve the application of the name Fusarium in the way it has been used for almost a century. Due to recent changes in the International Code of Nomenclature for algae, fungi, and plants, this is an urgent matter that requires community attention. The alternative is to break the longstanding concept of Fusarium into nine or more genera, and remove important taxa such as those in the F. solani species complex from the genus, a move we believe is unnecessary. Here we present taxonomic and nomenclatural proposals that will preserve established research connections and facilitate communication within and between research communities, and at the same time support strong scientific principles and good taxonomic practice.

Evolutionary Relationships among the Fusarium oxysporum f. sp. cubense Vegetative Compatibility Groups

ABSTRACT Fusarium oxysporum f. sp. cubense, the causal agent of fusarium wilt of banana (Musa spp.), is one of the most destructive strains of the vascular wilt fungus F. oxysporum. Genetic relatedness among and within vegetative compatibility groups (VCGs) of F. oxysporum f. sp. cubense was studied by sequencing two nuclear and two mitochondrial DNA regions in a collection of 70 F. oxysporum isolates that include representatives of 20 VCGs of F. oxysporum f. sp. cubense, other formae speciales, and nonpathogens. To determine the ability of F. oxysporum f. sp. cubense to sexually recombine, crosses were made between isolates of opposite mating types. Phylogenetic analysis separated the F. oxysporum isolates into two clades and eight lineages. Phylogenetic relationships between F. oxysporum f. sp. cubense and other formae speciales of F. oxysporum and the relationships among VCGs and races of F. oxysporum f. sp. cubense clearly showed that F. oxysporum f. sp. cubenses ability to cause disease on banana has emerged multiple times, independently, and that the ability to cause disease to a specific banana cultivar is also a polyphyletic trait. These analyses further suggest that both coevolution with the host and horizontal gene transfer may have played important roles in the evolutionary history of the pathogen. All examined isolates harbored one of the two mating-type idiomorphs, but never both, which suggests a heterothallic mating system should sexual reproduction occur. Although, no sexual structures were observed, some lineages of F. oxysporum f. sp. cubense harbored MAT-1 and MAT-2 isolates, suggesting a potential that these lineages have a sexual origin that might be more recent than initially anticipated.

Systematic Numbering of Vegetative Compatibility Groups in the Plant Pathogenic Fungus Fusarium oxysporum

First author: Plant Pathology Department, University of Florida, Gainesville 32611-0680; second author: Laboratoire de Recherches sur la Flore Pathogene du Sol, INRA, 17 rue Sully, B.V. 154

Occurrence and Pathogenicity of Fungi Associated with Melon Root Rot and Vine Decline in California

The occurrence of fungi associated with root rot and vine decline of melon (Cucumis melo) in commercial fields in California was surveyed over 3 years. The fungi most frequently isolated from discolored vascular tissue or root rot were Acremonium cucurbitacearum, Rhizopycnis vagum, Monosporascus cannonballus, Fusarium solani, Macrophomina phaseolina, Pythium spp., and Verticillium dahliae. The frequency of isolation of the various fungi varied with root symptomology. Pythium spp., and M. phaseolina were frequently associated with a wet, brownish root rot, while A. cucurbitacearum, R. vagum, and Rhizoctonia solani were generally associated with a dry, corky root rot. Presence of Monosporascus cannonballus was associated both with a wet, brownish rot as well as with discrete, reddish, corky lesions. The frequency of isolation of a given pathogen varied with geographic location, with M. cannonballus present only in the southern production areas, while A. cucurbitacearum and Rhizopycnis vagum were most common in the northern production areas. In pathogenicity tests in field microplots, M. cannonballus caused vine collapse and severe root rot of cantaloupe, reducing root length density by 93%. California isolates of R. vagum and A. cucurbitacearum, although only weakly pathogenic in field microplots, caused root discoloration and reduced vine growth in greenhouse tests. Reduction in dry weight of greenhouse-grown cantaloupe was 40, 23, and 39% for R. vagum, A. cucurbitacearum, and M. cannonballus, respectively.

Gibberella fujikuroi mating population A and Fusarium subglutinans from teosinte species and maize from Mexico and Central America.

Seed samples of maize ( Zea mays ssp. mays ) from Mexico and of teosintes ( Zea spp.), the nearest wild relatives of maize, from Mexico, Guatemala, and Nicaragua were assessed for infection with Fusarium species. Strains similar in morphology to Fusarium moniliforme and F. subglutinans were the most frequent isolates from maize and from teosinte species including Z. diploperennis, Z. luxurians, Z. mays ssp. mexicana , and Z. mays ssp. parviglumis . Analysis of fertility, vegetative compatibility and mycotoxin production identified 63 % of the 70 F. moniliforme strains from teosinte as genetically diverse members of Gibberella fujikuroi mating population A, a common pathogen of maize. The F. subglutinans strains from maize and teosinte were similarly genetically diverse, but were not fertile with standard testers of G. fujikuroi mating populations B and E, common pathogens of Poaceae , or of mating population H, which causes pitch canker disease of pine. Fifty-four percent of the 80 F. subglutinans strains were fertile when crossed with female tester strains from teosinte and maize collected in a field at Netzahualcoyotyl in the state of Mexico. These strains from Mexico and Central America may comprise a new and distinct G. fujikuroi mating population, but a strain from the Netzahualcoyotyl field site was fertile with a strain of G. fujikuroi mating population H from California. Thus, F. subglutinans from teosinte and maize may have a close relationship to mating population H from pine.

Beyond Yield: Plant Disease in the Context of Ecosystem Services

The ecosystem services concept provides a means to define successful disease management more broadly, beyond short-term crop yield evaluations. Plant disease can affect ecosystem services directly, such as through removal of plants providing services, or indirectly through the effects of disease management activities, including pesticide applications, tillage, and other methods of plant removal. Increased plant biodiversity may reduce disease risk if susceptible host tissue becomes less common, or may increase risk if additional plant species are important in completing pathogen life cycles. Arthropod and microbial biodiversity may play similar roles. Distant ecosystems may provide a disservice as the setting for the evolution of pathogens that later invade a focal ecosystem, where plants have not evolved defenses. Conversely, distant ecosystems may provide a service as sources of genetic resources of great value to agriculture, including disease resistance genes. Good policies are needed to support conservation and optimal use of genetic resources, protect ecosystems from exotic pathogens, and limit the homogeneity of agricultural systems. Research is needed to provide policy makers, farmers, and consumers with the information required for evaluating trade-offs in the pursuit of the full range of ecosystem services desired from managed and native ecosystems.