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Ecologically Based Approaches to Management of Phytophthora Blight on Bell Pepper

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Ecologically Based Approaches to Management of Phytophthora Blight on Bell Pepper Jean Beagle Ristaino North Carolina State University, Raleigh Stephen A. Johnston Rutgers Agricultural Research and Extension Center, Bridgeton, NJ 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 Ecologically Based 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 Approaches to when inoculum is splash dispersed from the soil to lower portions of the plant. The pathogen can also infect fruit and causes Management lesions that are typically covered with white sporangia, a sign of the pathogen (Fig. 4). P. capsici typically causes a fruit of Phytophthora Blight rot or stem rot on cucumbers and squash (Fig. 5). P. capsici reproduces by both sexual and asexual means (Fig. 6). The pathogen pro- on Bell Pepper duces 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 hor- mones that are responsible for gametangia differentiation in the opposite mating type. Both A1 and A2 mating types are common Phytophthora blight, caused by the oo- input of knowledge concerning a pathogen in fields in North Carolina and have also mycete pathogen, Phytophthora capsici, is life cycle, and secondarily, when necessary, been identified within the same plant (59). a devastating disease on bell pepper and on physical, chemical, and biological sup- P. capsici produces a male gametangium, cucurbit crops in the United States and plements for disease management. An un- called the antheridium, and a female worldwide (29,40). P. capsici causes a root derstanding of the ecological processes that gametangium, called the oogonium. The and crown rot, as well as an aerial blight of are suppressive to plant diseases is empha- antheridium is amphigynous in this spe- leaves, fruit, and stems, on bell pepper sized rather than secondary inputs (25). cies. Meiosis occurs within the gametan- (Capsicum annuum), tomatoes, cucumber, Fortunately, we have a considerable gia, and plasmogamy and karyogamy result watermelon, squash, and pumpkin (29,35, amount of information available on the 40,57,73). The disease was first described biology and ecology of P. capsici, which on bell pepper in New Mexico in 1922 can now be integrated to improve our abil- (40). In recent years, epidemics have been ity to manage the disease using ecologi- severe in areas of North Carolina, Florida, cally based approaches. Strategies recom- Georgia, Michigan, and New Jersey. Oo- mended for management of Phytophthora spores are believed to provide the initial blight involve integrated approaches that source of inoculum in the field, and the focus first on cultural practices that reduce disease is polycyclic within seasons high soil moisture conditions, but also (1,7,59,60,67). In this article, we discuss include monitoring and reduction of propa- the biology and epidemiology of Phy- gules of P. capsici that persist in the soil, tophthora blight on bell pepper and also utilization of cultivars with resistance to describe management strategies that can be the disease, and when necessary, judicious implemented based on existing knowledge fungicide applications. 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 Dr. Ristaino’s address is: Box 7616 Department of Plant Pathology, North Carolina State University, Raleigh 27695; E-mail: [email protected] Publication no. D-1999-0927-01F Fig. 1. Root and crown rot on bell pep- Fig. 2. Black stem lesion caused by Phy- © 1999 The American Phytopathological Society per caused by Phytophthora capsici. tophthora capsici on bell pepper. 1080 Plant Disease / Vol. 83 No. 12 in the formation of oospores, which are the in saturated soil (3). Zoospores can move propagule types from soil, but it is not sexual spores that serve as the overwinter- readily under saturated conditions and sufficiently quantitative to estimate inocu- ing inoculum of the pathogen (Fig. 7). The infect roots or aboveground portions of lum densities (38). Infected leaf disks are diploid nature of the oomycetes was docu- plants (18,19). plated onto semiselective media, and the mented in early studies on the genetics of presence of colonies of the pathogen al- P. capsici (66). Oospores can germinate Detection and Quantification lows qualitative determination of the pres- either directly via formation of a germ tube Traditionally, P. capsici has been recov- ence of the pathogen in the soil or water or indirectly via sporangium formation ered by isolation of the pathogen from sample. Soil dilution plating directly onto (Fig. 6). infected tissue and plating onto a semise- semiselective media without prior sample The pathogen also reproduces asexually lective medium (32,41). The pathogen can incubation is useful for the quantification via sporangia that are borne on branched also be isolated from soil following soil of zoospores and sporangia in soil, whereas sporangiophores (40,69). Sporangia are dilution plating in dilute water agar onto a soil dilution plating after sample saturation generally ovoid and have a prominent pa- semiselective medium such as Masago’s and incubation allows recovery of oospores pillum at their apex (Fig. 8). Sporangia are medium (Fig. 9) (41). No single assay from soil (38). The incubation period pro- easily dislodged from the sporangiophore method is suitable for the accurate detec- vides an opportunity for oospores to ger- and can be dispersed within fields by wind, tion and quantification of all propagule minate and produce sporangia and zoo- rain, and irrigation water (8,11,61). Spo- types of P. capsici in soil (38). A leaf disk spores that are then detected by the dilution rangia germinate indirectly and release baiting assay in which circular disks of plate assay. Many of the traditional soil motile, biflagellate zoospores under condi- pepper leaves are floated on saturated soil assays are laborious and time-consuming tions of free moisture on plant surfaces or provides high levels of detection of all and require replication to reduce sampling variation. Traditionally, accurate identification of the pathogen required morphological ob- servations of sporangiophores, sporangia, zoospore release, and oospores (69). Iden- tification using classical taxonomic meth- ods is time-consuming and requires exper- tise. Isozyme analysis, serological assays, DNA probes, restriction fragment length polymorphism (RFLP), and polymerase chain reaction (PCR) methods have been used to identify and evaluate the genetic variation among Phytophthora species (16,20,28,39,45,46,50). The nuclear small subunit rDNA sequences evolve relatively slowly and are useful for studying distantly related organisms, whereas the internal transcribed spacer (ITS) regions and inter- genic region of the nuclear rRNA repeat units vary among species and populations (74). Sequencing studies with small subunit ribosomal RNA sequences (20) have elucidated the evolutionary lineage of Fig. 3. Grayish brown water-soaked lesions on leaves of bell pepper caused by Phy- the oomycetes. Phytophthora species are tophthora capsici. actually more closely related to nonphoto- Fig. 4. Lesion caused by Phytophthora capsici with white mycelium and sporangia of the pathogen on the bell pepper fruit. Fig. 5. Phytophthora blight lesions on fruit of cucumbers and squash. Plant Disease / December 1999 1081 synthetic algae than to true fungi or land 140 bp, are present in P. capsici after di- studies are also needed to relate levels of plants and have been placed in the phylo- gestion with MspI (Fig. 11, lanes 5). P. propagules of the pathogen to levels of genetic assemblage, stramenopile (14,24). capsici can be differentiated from P. DNA recovered from soil. The nuclear small subunit rDNA and citrophthora by restriction digest of ITS ITS sequences have been used to develop DNA with RsaI (64). Impact of Soil Water rapid tools for identification of the eco- PCR assays should make diagnosis of P. Soil moisture, especially the matric nomically important species within the capsici in infected plant material, soil, or component of soil water potential (ψm), genus Phytophthora (39,64). A primer water samples more accurate. PCR is more plays an important role in the life cycle of called PCAP was developed based on se- rapid and efficient than traditional isolation Phytophthora spp. that cause root rots, quence data published previously for a P. methods in identification of P. capsici in including P. capsici (18,19). Sporangia of capsici-specific DNA probe (39,64). PCAP infected plant samples. We tested the PCR P. capsici form abundantly in saturated soil is used with ITS 1 in PCR reactions and assay and found that 92% of infected pep- within 24 h on mycelial disks that are first amplifies an approximately 172-bp frag- per plants with visible lesions that were incubated at drier ψm of –20.0 to –30.0 ment of DNA in all isolates of P. capsici positive by traditional isolation on media J/kg (1 J/kg = 10 millibars; a ψm value of 0 tested from a range of hosts (Fig. 10, lanes were also positive by PCR using the PCAP is fully saturated soil) (3). Release of zoo- 2 to 9 and 12). Isolates of P. citrophthora and ITS 1 primers (64). PCR with these spores from sporangia in Phytophthora (Fig. 10, lane 10) are also amplified by the primers also detected infection in 32% of species is extremely sensitive to small PCAP primer, but the amplified product the samples where the pathogen was not changes in ψm and occurs predominantly at size is larger than that of P.
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