SUMMARY
Phytophthora capsici is a highly dynamic and destructive pathogen of vegetables. It attacks all cucurbits, pepper, tomato and eggplant, and, more recently, snap and lima beans. The disease ...incidence and severity have increased significantly in recent decades and the molecular resources to study this pathogen are growing and now include a reference genome. At the population level, the epidemiology varies according to the geographical location, with populations in South America dominated by clonal reproduction, and populations in the USA and South Africa composed of many unique genotypes in which sexual reproduction is common. Just as the impact of crop loss as a result of P. capsici has increased in recent decades, there has been a similar increase in the development of new tools and resources to study this devastating pathogen. Phytophthora capsici presents an attractive model for understanding broad‐host‐range oomycetes, the impact of sexual recombination in field populations and the basic mechanisms of Phytophthora virulence.
Taxonomy: Kingdom Chromista; Phylum Oomycota; Class Oomycetes; Order Peronosporales; Family Peronosporaceae; Genus Phytophthora; Species capsici.
Disease symptoms: Symptoms vary considerably according to the host, plant part infected and environmental conditions. For example, in dry areas (e.g. southwestern USA and southern France), infection on tomato and bell or chilli pepper is generally on the roots and crown, and the infected plants have a distinctive black/brown lesion visible at the soil line (Fig. 1). In areas in which rainfall is more common (e.g. eastern USA), all parts of the plant are infected, including the roots, crown, foliage and fruit (Fig. 1). Root infections cause damping off in seedlings, whereas, in older plants, it is common to see stunted growth, wilting and, eventually, death. For tomatoes, it is common to see significant adventitious root growth just above an infected tap root, and the stunted plants, although severely compromised, may not die. For many cucurbit fruit, the expanding lesions produce fresh sporangia over days (or even weeks depending on the size of the fruit) and the fruit often look as if they have been dipped in white powdered confectioner's sugar (Fig. 1). Generally, hyphae do not emerge from infected plants or fruit (common with Pythium infections) and all that is visible on the surface of an infected plant is sporangia.
1
Symptoms associated with Phytophthora capsici infection on various hosts. (A) Tomato plant with root and crown rot caused by P. capsici; note the distinctive brown lesion on the lower part of the plant. (B) Tap root of pumpkin plant showing loss of roots and browning caused by infection. (C) Infected bell pepper with spores on the surface of the fruit. (D) Heavy sporangia production on the surface of infected cucumber fruit. (E) Assortment of ornamental and edible winter squash infected with P. capsici in Tennessee. (F) Natural lesion exhibiting a clearly demarcated island of tissue that contains biotrophic infection surrounded by necrotic tissue with sporangia.
Importance: Phytophthora capsici presents an oomycete worst‐case scenario to growers as it has a broad host range, often produces long‐lived dormant sexual spores, has extensive genotypic diversity and has an explosive asexual disease cycle. It is becoming increasingly apparent that novel control strategies are needed to safeguard food production from P. capsici and other oomycetes. Considering that P. capsici is easy to grow, mate and manipulate in the laboratory and infects many plant species, this pathogen is a robust model for investigations, particularly those related to sexual reproduction, host range and virulence.
Useful websites: Phytophthora capsici genome database: http://genome.jgi‐psf.org/Phyca11/Phyca11.home.html. Molecular tools to identify Phytophthora isolates: http://phytophthora‐id.org.
is the most devastating pathogen for chile pepper production worldwide and current management strategies are not effective. The population structure of the pathogen is highly variable and few sources ...of widely applicable host resistance have been identified. Recent genomic advancements in the host and the pathogen provide important insights into the difficulties reported by epidemiological and physiological studies published over the past century. This review highlights important challenges unique to this complex pathosystem and suggests strategies for resistance breeding to help limit losses associated with
.
.
Many viruses occur in apple (
(Borkh.)), but no information is available on their seed transmissibility. Here, we report that six viruses infecting apple trees, namely, apple chlorotic leaf spot ...virus (ACLSV), apple green crinkle-associated virus (AGCaV), apple rubbery wood virus 2 (ARWV2), apple stem grooving virus (ASGV), apple stem pitting virus (ASPV), and citrus concave gum-associated virus (CCGaV) occur in seeds extracted from apple fruits produced by infected maternal trees. Reverse transcription polymerase chain reaction (RT-PCR) and quantitative RT-PCR (RT-qPCR) assays revealed the presence of these six viruses in untreated apple seeds with incidence rates ranging from 20% to 96%. Furthermore, ASPV was detected by RT-PCR in the flesh and peel of fruits produced by infected maternal trees, as well as from seeds extracted from apple fruits sold for fresh consumption. Finally, a large-scale seedling grow-out experiment failed to detect ACLSV, ASGV, or ASPV in over 1000 progeny derived from sodium hypochlorite surface sterilized seeds extracted from fruits produced by infected maternal trees, suggesting no detectable transmission via embryonic tissue. This is the first report on the seedborne nature of apple-infecting viruses.
The oomycete vegetable pathogen Phytophthora capsici has shown remarkable adaptation to fungicides and new hosts. Like other members of this destructive genus, P. capsici has an explosive ...epidemiology, rapidly producing massive numbers of asexual spores on infected hosts. In addition, P. capsici can remain dormant for years as sexually recombined oospores, making it difficult to produce crops at infested sites, and allowing outcrossing populations to maintain significant genetic variation. Genome sequencing, development of a high-density genetic map, and integrative genomic or genetic characterization of P. capsici field isolates and intercross progeny revealed significant mitotic loss of heterozygosity (LOH) in diverse isolates. LOH was detected in clonally propagated field isolates and sexual progeny, cumulatively affecting >30% of the genome. LOH altered genotypes for more than 11,000 single-nucleotide variant sites and showed a strong association with changes in mating type and pathogenicity. Overall, it appears that LOH may provide a rapid mechanism for fixing alleles and may be an important component of adaptability for P. capsici.
The oomycetes form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms that are relatively closely related to photosynthetic algae such as brown algae and diatoms. Plant ...pathogenic species, notably those of the genus Phytophthora, are the best-studied oomycetes. The genomes of four Phytophthora and one downy mildew species were recently sequenced resulting in novel insights on the evolution and pathogenesis of oomycetes. This review highlights key findings that emerged from these studies and discusses the future challenges for oomycete research.
Phytophthora capsici
is a devastating disease of pepper (
Capsicum
sp.) in Taiwan causing complete loss of commercial fields. The objective of this study was to characterize genetic diversity for 38 ...newly collected isolates and three historical isolates. Analysis of data includes whole genome sequence for two new isolates and for two isolates collected previously in 1987 and 1995. In addition, 63 single nucleotide polymorphism loci were genotyped using targeted-sequencing, revealing 27 unique genotypes. Genotypes fell into three genetic groups: two of the groups contain 90% (
n
= 33) of the 2016 isolates, are triploid (or higher), are exclusively the A2 mating type and appear to be two distinct clonal lineages. The isolates from 2016 that grouped with the historical isolates are diploid and the A1 mating type. Whole genome sequence revealed that ploidy varies by linkage group, and it appears the A2 clonal lineages may have switched mating type due to increased ploidy. Most of the isolates were recently race-typed on a set of differential
C. annuum
, and although there was no direct correlation between virulence and ploidy, many of the triploid isolates were less virulent as compared to the historical diploid isolates. The implications for breeding resistant pepper and conducting population analyses are discussed.
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