Epidemiology and management of ascochyta blight of field pea (Pisum sativum) in South Australia.
Date
2012
Authors
Davidson, Jennifer Anne
Editors
Advisors
Scott, Eileen Sandra
Salam, Moin
Salam, Moin
Journal Title
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Thesis
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Abstract
Ascochyta blight disease (synonym: blackspot) of field pea has worldwide distribution and
regularly causes AUD$25 million loss per annum in Australian field pea (Pisum sativum)
crops. This study provides new information on the causal pathogens and management
strategies to reduce loss from this disease.
Research involving sowing dates, genotypes and fungicide treatments was conducted
to identify optimal management strategies. Earlier sowing generally resulted in higher yield
except when ascochyta blight was severe. Yield response to fungicide application varied with
disease severity, sowing date and genotype. The optimum sowing period was within a week
of the first autumn rains in low rainfall regions and 3 weeks after the first autumn rains in
medium and medium - high rainfall regions. Earlier flowering genotypes were the highest
yielding particularly when sown early and subjected to strategic fungicide applications.
The pathogen, Phoma koolunga, was recognised for the first time as a component of
the ascochyta blight disease complex in southern Australia. The species was described
morphologically. Sequences of the internal transcribed spacer region were distinct from those
of the accepted causal pathogens of ascochyta blight of field pea viz. Didymella pinodes,
Phoma medicaginis var. pinodella and Ascochyta pisi. Symptoms on field pea seedlings
caused by P. koolunga were indistinguishable from those caused by D. pinodes, other than a
24 h delay in manifestation of symptoms.
P. koolunga was detected across field pea cropping soils in South Australia but rarely
from other Australian states while D. pinodes plus P. medicaginis var. pinodella were
widespread. The quantity of DNA of these pathogens detected in soils was positively
correlated with ascochyta blight lesions in a pot bioassay. Soil-borne inoculum gradually
decreased in the 3 years following a field pea crop. DNA tests and pathogen isolation from
naturally infected field pea plants showed P. koolunga to be an important component of the disease complex in South Australia. P. koolunga and D. pinodes were equally responsible for
disease symptoms, while P. medicaginis var. pinodella had a minor role in the disease
complex.
Interaction between D. pinodes, P. medicaginis var. pinodella and P. koolunga was
investigated in controlled conditions. Colony diameter of the former was reduced on potato
dextrose agar (PDA) amended with filtrate from broth cultures of P. koolunga, as was colony
diameter of D. pinodes on PDA amended with filtrate from P. medicaginis var. pinodella or
D. pinodes. This effect was shown to be fungistatic rather than fungicidal. When coinoculated
onto leaves on field pea plants, or onto excised leaf discs, either the quantity of
DNA of D. pinodes and of P. medicaginis var. pinodella, or the mean lesion diameter of these
pathogens, was significantly reduced when co-inoculated with P. koolunga. P. koolunga was
not influenced by co-inoculation. D. pinodes demonstrated self-antagonism.
D. pinodes is considered the principal pathogen of concern in this complex. This study
further investigated the relationship between ascospore numbers of D. pinodes at sowing and
disease at the end of the season. Ascospores released from stubble infested with ascochyta
blight were counted periodically in a wind tunnel. A model was developed to predict disease
severity in relation to ascospore numbers, distance from infested field pea stubble, and
rainfall. The model was validated with an independent dataset. A threshold level of
ascospores of D. pinodes was identified above which disease did not increase.
The findings from this study have been incorporated into management
recommendations for field pea in southern Australia. Growers are encouraged to manipulate
sowing dates according to the temporal release of ascospores, and select a cultivar that has the
best agronomic yield potential for the sowing date, and to use fungicide strategically. The
recommendation also emphasises field selection based on commercial testing for the presence
of soil-borne inoculum of D. pinodes, P. medicaginis var. pinodella and P. koolunga.
School/Discipline
School of Agriculture, Food and Wine
Dissertation Note
Thesis(Ph.D.)-- University of Adelaide, School of Agriculture, Food and Wine, 2012.
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.