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Epidemiology of Botrytis spp. associated with neck rot of onion (Allium cepa L.) in northern Tasmania, Australia

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Chilvers, Martin Ian (2003) Epidemiology of Botrytis spp. associated with neck rot of onion (Allium cepa L.) in northern Tasmania, Australia. PhD thesis, University of Tasmania.

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Abstract

Neck rot of onion, caused by a complex of Botrytis spp., is an important fungal
disease of onion worldwide. In Tasmania, it has caused considerable losses to the
onion industry in some seasons. Botrytis infects the onion plant in the field but usually
the infection remains symptomless, with the fungus growing into the bulb during
curing and producing a rot of the bulb in storage. The taxonomy of Botrytis species
causing onion neck rot is currently under review. Recent studies of the ribosomal
internal transcribed spacer (ITS) region of the genome of Botrytis spp. associated with
neck rot have confirmed the existence of three distinct groups. A preliminary study of
24 isolates' from Australia identified one isolate as B. aclada type Al, and 22 isolates
as.B. aclada type All (B. allii) with no isolates of B. byssoidea found.
A survey of 16 commercial onion bulb crops in northern Tasmania from October to
December of 1999 detected B. allii in leaf samples from six crops at an incidence of
0.1% to 0.3%. In three crops that were re-sampled in January 2000, incidence had
increased from 0.1% to 3.2%, 0% to 0.5% and 0.3% to 5.8%. Infected crops were
found in both main onion-growing regions (northwest and the northeast) in Tasmania.
After storage of bulb samples from eight of the surveyed crops, the incidence of
Botrytis neck rot ranged from 0.4% to 16.3% with an average incidence of 5.3%.
Two field trials were conducted to examine the spatio-temporal spread of B. allii
during the season from a point source (2000) and line source (2001). Data of B.
cinerea spatio-temporal spread was also collected. In 2000, 100 plots (1.60 by 4.48 m)
• were established in a 10-by-10 lattice. Leaf samples were taken six times during the
growing season and incubated under high humidity to determine the incidence of B.
allii in foliage. The incidence of B. allii increased from 0.0% at placement of
inoculum (150 days after sowing) to a cumulative 3.8% and a non-cumulative 1.8% at
84 days after placement of inoculum (234 days after sowing). The cumulative sample
method was based on always treating a plant as infected once it was found to be
infected. The non-cumulative sampling method was the incidence found at each
sample time. The disease progress curve was described best by exponential and
logistic models (P = 0.001, R2 = 0.99), indicating secondary spread of the pathogen.
The disease gradient was steep and best described by an exponential model, usually
indicative of a rain splash-dispersed pathogen. Spatial analysis using ordinary runs
analysis, beta-binomial distribution analysis, spatial analysis by distance indices and
radial correlation analysis determined that the pattern of infection was aggregated.

Item Type: Thesis (PhD)
Keywords: Botrytis, Onion industry, Onions
Copyright Holders: The Author
Copyright Information:

Copyright 2003 the Author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2003. Includes bibliographical references

Date Deposited: 25 Nov 2014 00:52
Last Modified: 11 Mar 2016 05:54
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