Lifestyles of Colletotrichum acutatum.

ilamentous fungi of the genus Colletotrichum and its teleomorph Glomerella are considered major plant pathogens worldwide. They cause significant economic damage to crops in tropical, subtropical, and temperate regions. Cereals, legumes, ornamentals, vegetables, and fruit trees may be seriously affected by the pathogen (3). Although many cultivated fruit crops are infected by Colletotrichum species, the most significant economic losses are incurred when the fruiting stage is attacked. Colletotrichum species cause typical disease symptoms known as anthracnose, characterized by sunken necrotic tissue where orange conidial masses are produced. Anthracnose diseases appear in both developing and mature plant tissues (4). Two distinct types of diseases occur: those affecting developing fruit in the field (preharvest) and those damaging mature fruit during storage (postharvest). The ability to cause latent or quiescent infections has grouped Colletotrichum among the most important postharvest pathogens. Species of the pathogen appear predominantly on aboveground plant tissues; however, belowground organs, such as roots and tubers, may also be affected. In this article, we deal in particular with methods used to identify and characterize Colletotrichum species and genotypes from almond, avocado, and strawberry, as examples, using traditional and molecular tools. The three pathosystems chosen represent different disease patterns of fruitassociated Colletotrichum. Multiple Species on a Single Host Numerous cases have been reported in which several Colletotrichum species or biotypes are associated with a single host. For example, avocado and mango anthracnose, caused by both C. acutatum and C. gloeosporioides, affect fruit predominantly as postharvest diseases (25,40,41). Strawberry may be infected by three Colletotrichum species, C. fragariae, C. acutatum, and C. gloeosporioides, causing anthracnose of fruit and other plant parts (31). Almond and other deciduous fruits may be infected by C. acutatum or C. gloeosporioides (Table 1) (1,5,46,50). Citrus can be affected by four different Colletotrichum diseases (61): postbloom fruit drop and key lime anthracnose, both caused by C. acutatum, and shoot dieback and leaf spot, and postharvest fruit decay, both caused by C. gloeosporioides. Additional examples of hosts affected by multiple Colletotrichum species include coffee, cucurbits, pepper, and tomato. Single Species on Multiple Hosts It is common to find that a single botanical species of Colletotrichum infects multiple hosts. For example, C. gloeosporioides (Penz.) Penz. & Sacc. in Penz. (teleomorph: Glomerella cingulata (Stoneman) Spauld. & H. Schrenk), which is considered a cumulative species and forms the sexual stage in some instances, is found on a wide variety of fruits, including almond, avocado, apple, and strawberry (Table 2) (6,15,31,46). Likewise, C. acutatum J.H. Simmonds has been reported to infect a large number of fruit crops, including avocado, strawberry, almond, apple, and peach (1,5,16,25,27). Examples of other species with multiple host ranges include C. coccodes, C. capsici, and C. dematium (14,56).

Over the last 10 years plant pathologists have begun to realize that more knowledge about the genetic structure of populations of plant pathogens is needed to implement effective control strategies (48). Research on the genetic structure of fungal populations has mushroomed, and review papers that summarize these studies are numerous (7,27,33,34,38). Although the number of fungal studies has increased greatly, the most comprehensive work has focused on a small number of plant-pathogenic fungi. The majority of these fungi can be recognized easily by their fruiting bodies or disease symptoms on aboveground plant parts. It has proven more difficult to assess the genetic structure of fungal populations that exist mainly belowground, because the distribution of individuals cannot be visualized directly and appropriate sampling procedures are less obvious and more cumbersome. Nevertheless, substantial progress has been made in interpreting the population genetic structure of some soilborne fungi (1,17). The purpose of this paper is to provide an overview of the tools and techniques of fungal population genetics. I will try to emphasize approaches that may be applied to studies of soilborne fungi. Instead of providing detailed methods, I will cite recent references where appropriate. There are many opinions regarding which techniques and tools are best suited to studies of fungal populations. I will give a personal and biased viewpoint, which I believe will be most useful to those who are just entering the field.

Fumonisins in Maize: Can We Reduce Their Occurrence?

erhaps the most widely used agents of biological plant protection are endophytic fungal symbionts (endophytes) of forage and turfgrasses. These are fungi of the family Clavicipitaceae, which grow between host cells in vegetative tissues, ovules, and seeds of systemically infected grass plants. The existence of these endophytes was not fully appreciated until recent years, although the protection they provide against insect damage (Fig. 1) and drought contributes to the superior agronomic qualities of favorite pasture grasses in North America, Australia, and New Zealand. Unfortunately for livestock farmers, these endophytes also provide a degree of protection from grazing mammals. In 1977, Bacon et al. (2) reported that the grass Festuca arundinacea var. genuina Schreb. (hexaploid tall fescue) had a fungal endophyte related to Epichloe typhina (Pers.:Fr.) Tul., and that this endophyte— now known as Neotyphodium coenophialum (Morgan-Jones & Gams) Glenn, Bacon, & Hanlin—was responsible for toxicosis suffered by livestock grazing the grass. Epichloe species were known for many decades (44), but reports relating that nonpathogenic endophytes could be detrimental to livestock provided new impetus for intensive studies, making the grass–endophyte associations among the best characterized symbioses in biology. Less than two decades of research have yielded a rich body of knowledge about these symbioses: their secondary product chemistry, ecology, evolution, genetics, and molecular biology; their ecological roles as protectants from insect and vertebrate herbivores, pathogenic fungi and nematodes, and drought; and their effects on host growth and competitiveness. The endophytes produce numerous alkaloids, some of which are unrelated to any known from plants or other fungi. Their genetic and evolutionary complexity is extraordinary. They were the first fungi genetically documented as interspecific hybrids (47,53). Meanwhile, molecular genetic techniques were applied to the endophytes of tall fescue and other grasses, bringing us closer to the prospect of reducing or eliminating their toxicosis to livestock while continuing to employ their bioprotective qualities.

The most important thermal properties of typical rocks and fluids encountered in thermal recovery operations and the variation of such properties with temperature, and in the case of rocks, with fluid saturation are discussed. An attempt will be made to cover the available experimental data, and the most acceptable calculation techniques which are suitable for both computer and hand computations. The thermal properties of liquids and gases which may be of interest to petroleum engineers engaged in thermal recovery operations are viscosity, density, thermal conductivity, specific heat, heat of vaporization, and vapor pressure. Some of these properties will be of greater interest in the case of liquids and rocks, than in the case of gases. (27 refs.)

Phytophthora blight, caused by the oomycete pathogen, Phytophthora capsici, is a devastating disease on bell pepper and cucurbit crops in the United States and worldwide (29,40). P. capsici causes a root and crown rot, as well as an aerial blight of leaves, fruit, and stems, on bell pepper (Capsicum annuum), tomatoes, cucumber, watermelon, squash, and pumpkin (29,35, 40,57,73). The disease was first described on bell pepper in New Mexico in 1922 (40). In recent years, epidemics have been severe in areas of North Carolina, Florida, Georgia, Michigan, and New Jersey. Oospores are believed to provide the initial source of inoculum in the field, and the disease is polycyclic within seasons (1,7,59,60,67). In this article, we discuss the biology and epidemiology of Phytophthora blight on bell pepper and also describe management strategies that can be implemented based on existing knowledge of the ecology of this devastating pathogen. The objectives of ecologically based pest management (EBPM) are the safe, profitable, and durable management of pests that includes a total systems approach (25). EBPM relies primarily on biological input of knowledge concerning a pathogen life cycle, and secondarily, when necessary, on physical, chemical, and biological supplements for disease management. An understanding of the ecological processes that are suppressive to plant diseases is emphasized rather than secondary inputs (25). Fortunately, we have a considerable amount of information available on the biology and ecology of P. capsici, which can now be integrated to improve our ability to manage the disease using ecologically based approaches. Strategies recommended for management of Phytophthora blight involve integrated approaches that focus first on cultural practices that reduce high soil moisture conditions, but also include monitoring and reduction of propagules of P. capsici that persist in the soil, utilization of cultivars with resistance to the disease, and when necessary, judicious fungicide applications. Symptoms and Life Cycle P. capsici can infect virtually every part of the pepper plant. The pathogen causes a root and crown rot on pepper (Fig. 1) and also forms distinctive black lesions on the stem (Fig. 2). P. capsici can also infect the leaves and causes lesions that are circular, grayish brown, and water-soaked (Fig. 3). Leaf lesions and stem lesions are common when inoculum is splash dispersed from the soil to lower portions of the plant. The pathogen can also infect fruit and causes lesions that are typically covered with white sporangia, a sign of the pathogen (Fig. 4). P. capsici typically causes a fruit rot or stem rot on cucumbers and squash (Fig. 5). P. capsici reproduces by both sexual and asexual means (Fig. 6). The pathogen produces two mating types, known as the A1 and A2. These are actually compatibility types and do not correspond to dimorphic forms. Each mating type produces hormones that are responsible for gametangia differentiation in the opposite mating type. Both A1 and A2 mating types are common in fields in North Carolina and have also been identified within the same plant (59). P. capsici produces a male gametangium, called the antheridium, and a female gametangium, called the oogonium. The antheridium is amphigynous in this species. Meiosis occurs within the gametangia, and plasmogamy and karyogamy result

Mango Anthracnose: Economic Impact and Current Options For Integrated Managaement.

Identification of breakpoints in intergenotypic recombinants of HIV type 1 by bootscanning.

The first report of the A2 mating (compatibility) type of the potato late blight pathogen, Phytophthora infestans (Mont.) de Bary, outside Mexico was in Europe during 1984 (32) and, since then, the A2 has been found in many parts of the world (19,23). The four most likely hypotheses to explain the occurrence of the A2 mating type outside Mexico are that it (i) was always present, but undetected (58); (ii) was introduced by migration (62); (iii) arose by mutation or mitotic recombination; or (iv) arose by mating type change, either from exposure to fungicides or by induced selfing (39). Among these, the migration hypothesis is the only one with strong scientific support. However, this subject remains somewhat controversial, and alternative explanations for the origin of the A2 mating type of P. infestans outside Mexico still appear occasionally in the literature. Analyses of allozyme data provided the first unambiguous evidence that the A2 mating type in Europe and Japan was introduced by migration from Mexico (62). Numerous additional population genetic studies have fully supported the migration hypothesis (13, 15,18,23,25,40,44,55,63). In each location studied, the first detection of A2 isolates coincided with the appearance of new alleles at allozyme, DNA fingerprint, and mitochondrial DNA loci. Similar changes occurred with the recent appearance of the A2 mating type in the United States (29). Although the detection of new alleles sometimes preceded the A2 (1,25), the A2 never appeared without new alleles (19,23). The migration hypothesis was challenged recently by Ko (39), who proposed instead that mating type change was the origin of the A2 mating type of P. infestans outside Mexico. Ko’s (39) conclusion came from his result that self-fertilization could initiate mating type change. Unfortunately, the mating type change hypothesis in P. infestans was not tested using genetic markers, and no genetic mechanism was proposed by which mating type change could occur. Furthermore, the population genetic data that contradict the mating type change hypothesis were ignored. Part of the proof for the mating type change hypothesis was based on a number of early literature reports that supposedly stated that homothallic isolates of P. infestans were present outside Mexico prior to the 1950s. Unfortunately, these early references were cited without critical evaluation. It is well-known that heterothallic species of Phytophthora produce occasional oospores in single culture (4,21,53,64, 65). It is also quite well documented that oospores of P. infestans were found occasionally during the early part of this century in Europe and the United States (8,48). However, because these structures were produced only rarely and under specific conditions, the scientific consensus was that the sexual stage of P. infestans remained to be discovered. This did not change until the A2 mating type was found in Mexico during the 1950s (21,47,61). Because of the strong evidence for migration, the mating type change hypothesis has never been tested directly. Fortunately, this hypothesis provides testable predictions about the genetic background that should be present in A2 isolates outside Mexico. If the A2 originated by mating type change from A1 mating type populations, the first A2 isolates in each location should be identical, or nearly identical except for mating type, to the previously existing A1 isolates. Sexual reproduction after mating type change could generate new genotypes, but they still should contain only the alleles present in the original A1 populations. Because most populations throughout the world, until recently, were composed primarily, or exclusively, of a single clonal lineage (15,23,25), the mating type change hypothesis is easily testable using molecular markers. Identical multilocus genotypes (or changes limited to a rearrangement of alleles) before and after the occurrence of the A2 mating type would confirm the mating type change hypothesis. In contrast, if the A2 mating type originated by migration, A2 isolates could be similar, or very distinct, from the original A1 isolates, depending on the source population for the migrating genotypes. If the first A2 isolates were very different from isolates in the old A1 populations, the mating type change hypothesis would be rejected. Our purpose was to reanalyze previously published genotypic data to explicitly test the mating type change hypothesis for the origin of the A2 of P. infestans outside Mexico. A secondary goal was to evaluate the early literature to test Ko’s (39) conclusion that homothallic isolates of P. infestans were known outside Mexico prior to the 1950s. Finally, mating type segregations in self-fertilized progenies of P. infestans were analyzed to determine whether mating type change has been observed by other investigators.

Pseudomonas syringae Diseases of Fruit Trees: Progress Toward Understanding and Control.

The pKNOCK series of broad-host-range mobilizable suicide vectors for gene knockout and targeted DNA insertion into the chromosome of gram-negative bacteria.

The biopesticide market for global agricultural use

Understanding the Mechanisms Employed by <i>Trichoderma virens</i> to Effect Biological Control of Cotton Diseases

Characterization and Recent Advances in Detection of Strawberry Viruses.

Sooty Blotch and Flyspeck of Apple: Etiology, Biology, and Control