Late wilt of corn - Magnaporthiopsis maydis
Effective: September 16, 2014
Taxonomic Position: Magnaporthales : Magnaporthaceae
Pest Type: Fungi
Pest Code (NAPIS): FGDNCGZ
This pest is a member of the following surveys: Corn
These Approved Methods are appropriate for: 2024, 2023, 2022, 2021, 2020, 2019, 2018, 2017, 2016, 2015
||NAPIS Survey Method
||Collect symptomatic plant material.
||3031 - General Visual Observation
The following are recommendations for executing the survey using the approved methods for pest surveillance. The recommendations are developed through literature review and consultation with subject matter experts.
No specific signs are present.
The first symptom is a moderately rapid wilting of plants usually at about the time of flowering until shortly before maturity. Wilting progresses from lower to upper portions of the plant. Leaves first become dull green, eventually lose color, and become dry as though suffering from lack of water. Vascular bundles in the stalk become reddish-brown, and lower internodes also assume this color. In advanced stages of the disease, lower portions of the stalk become dry, shrunken, and hollow. Secondary infection by other pathogens frequently progresses into stalk rot.
Key Diagnostic or Identification
Molecular: Two diagnostic assays have been developed for Harpophora maydis, causal agent of late wilt of maize, by CPHST Beltsville. These assays have been put through initial stages of validation and the conventional PCR assay has been tested by a collaborator in Israel, where H. maydis is an established plant pathogen. Work Instructions for both the conventional and qPCR assays have been written, and have been reviewed and approval for general use in CPHST.
Both of the conventional PCR and qPCR assays target a segment of the Ribosomal Internal Transcribed Spacer (ITS) of H. maydis . The conventional PCR assay has been optimized as a stand-alone (singleplex) assay. The ITS region of H. maydis has been cloned into a pCR2.1-TOPO vector and constitutes the positive control for this assay. The qPCR assay has been optimized as a stand-alone (singleplex) assay, and as a multiplex assay combining a diagnostic PCR assay specific for H. maydis , and an internal control (IC) assay targeting a known plant DNA locus (COX).
These Work Instructions were released for use in screening samples beginning in FY2014.
In Progress / Literature-based Diagnostics:
Morphological: Pathogen may be identified morphologically by examination of the shape and size of conidia and conidiophores, color and type of colony, and temperature requirements (Samra et al., 1963).
Culture the fungus in complete medium (CM) broth and incubate on an orbital shaker for 3 days at room temperature. Harvest mycelia for DNA extraction.
Molecular: Ward and Bateman (1999) use a pair of PCR primers that amplify a segment of the ribosomal gene locus from many members of the Gaeumannomyces and Phialophora fungal pathogens from maize and other host plants. The PCR product from Harpophora maydis (=Cephalosporium maydis) can be distinguished from that of other members of the group on the basis of its unique size (490 bp) relative to that of other species.
PCR primers have been developed for differentiating H. maydis (=Cephalosporium maydis) from some of its relatives (Acremonium, Cephalosporium, Gaeumannomcyes, and Phialophora) (Saleh and Leslie, 2004).
Symptoms can be confused with drought stress.
When culturing, the fungus may be confused with other more rapidly growing Harpophora and Gaeumannomyces species.