PEST RISK ANALYSIS (PRA) TRAINING Group Exercises Manual

PEST RISK ANALYSIS TRAINING - GROUP EXERCISES MANUAL GROUP EXERCISE NO. 2
Example No. 8 – Texas Root Rot 8
SCENARIO: Mexican authorities have stopped a shipment of carrots at the border. The carrots
were contaminated with soil, and preliminary investigations indicate that Texas root rot
(Phymatotrichopsis omnivora) may be a concern. Their department of agriculture (SAGARPA)
have initiated a PRA to determine whether the shipment should be allowed to proceed.
PEST DATASHEET:
Identity: Phymatotrichopsis omnivora (Duggar) Hennebert (Fungi: Basidiomycetes:
Aphyllophorales). Synonyms: Phymatotrichum omnivorum Duggar; Ozonium omnivorum Shear;
Ozonium auricomum Link. Telemorph: Possibly Trechispora brinkmanii (Bresad.) D. P. Rogers
and H. S. Jackson. (Despite some doubt on the association with a specific telemorph,
basiodiomycete affinities seem clear). Common name: Texas root rot; cotton root rot.
Hosts: The major host is cotton, including Gossypium herbaceum, G. hirsutum, and G.
barbadense. The fungus can also develop on more than 200 species of dicotyledonous plants
including 31 economic field crops, 58 vegetable crops, 18 fruits and berries including citrus, 35
forest trees and shrubs, 7 herbaceous ornamentals, and 20 weeds. Monocotyledons are thought to
be immune, but fungal strands have been reported on such hosts in nature.
Distribution: Origin: Northern Mexico and southwestern United States (Arizona, Arkansas,
California, Louisiana, Nevada, New Mexico, Oklahoma, Texas, Utah). Present: Reported from
Venezuela; Lybia. Absent: No confirmed reports from Europe, Asia, or the South Pacific; Africa
(except Lybia); most of Central and South America (except Venezuela); northern North America.
Biology and Dispersal: Sclerotia are the primary inoculum source for the initiation of disease.
They also serve as over-seasoning propagules that enable the pathogen to persist in soil for many
years. Strands have been shown to survive on dead cotton roots for at least one year, but their role
in initiating disease in subsequent seasons is not known. Contact with growing roots is made by
strands from germinating sclerotia or adjacent infected plants. Strands continue to proliferate on
the taproot and associated lesions can be found on lateral roots as the fungus spreads from the
taproot into the surrounding soil. The root periderm is penetrated (directly or through lenticels,
points of emergence of lateral roots or ruptures in the periderm) with hyphae invading and
spreading in the underlying cambium and xylem portions of the root. With field crops such as
cotton, sclerotia are formed following infection and plant death. These sclerotia serve as overseasoning
propagules and initiate infections the following growing season.
Damage: The fungus is most serious on cotton; on this host it kills plants before maturity, reduces
yield and reduces lint quality in plants which survive until harvest. In Texas, it is estimated that 2%
of the cotton yield is lost each year to root rot. Winter-grown annual crops (such as sugarbeet)
escape disease. Apples, peaches, pecans, grapes, and in Mexico, mangoes and avocados, suffer
significant losses. Ulmus spp. have been severely attacked, but the disease in only minor on citrus,
roses and Rhododendron. In general, there is relatively little information on hosts other than
cotton, suggesting that while many species may be infected, few suffer economic loss.
Control: For most crops and soil types, there are no control measures that are both effective and
economically justified. Soil fumigants such as 1,3-dichloropropene have been shown to provide
control of the pathogen; these treatments may be justified as pre-plant treatments for orchards or
vineyards but are cost prohibitive for annual row crops. The application of a systemic triazole
fungicide deep in the soil near the root appears to offer the potential for disease reduction.
Currently, other control measures (e.g. resistant varieties, biological, cultural) have not yielded
adequate or sufficiently consistent reduction in plant mortality to be useful.
8 NOTE: This example was developed specifically for use in the IPPC PRA training course. The
scenario is not real. The data sheet is based on the EPPO Data Sheet, URL: http://www.eppo.org/ and
CABI Crop Protection Compendium URL: http://www.cabi.org/compendia/cpc
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