Sugarbeet leaf spot disease (Cercospora beticola
Sacc.)
John Weiland1 and Georg Koch2
1 United States Department of Agriculture,
Agricultural Research Service, Northern Crop Science Laboratory, Fargo, ND
58105, USA
2 Strube-Dieckmann, A. Dieckmann-Heimburg, Postfach 1165, 31684 Nienstädt,
Germany
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| Summary: |
Leaf spot disease
caused by Cercospora beticola Sacc. is the most destructive foliar
pathogen of sugarbeet worldwide. In addition to reducing yield and quality
of sugarbeet, the control of leaf spot disease by extensive fungicide
application incurs added costs to producers and repeatedly has selected
for fungicide-tolerant C. beticola strains. The genetics and
biochemistry of virulence have been examined less for C. beticola
as compared with the related fungi C. nicotianae, C. kikuchii
and C. zeae-maydis, fungi to which the physiology of C. beticola
is often compared. C. beticola populations generally are not
characterized as having race structure, although a case of race-specific
resistance in sugarbeet to C. beticola has been reported.
Resistance currently implemented in the field is quantitatively inherited
and exhibits low to medium heritability. |
| Taxonomy: |
Cercospora beticola
Sacc.; Kingdom Fungi, Subdivision Deuteromycetes, Class Hyphomycetes,
Order Hyphales, Genus Cercospora. |
| Identification: |
Circular, brown to red
delimited spots with ashen-grey centre, 0.5-6 mm diameter; dark brown
to black stromata against grey background; pale brown unbranched sparingly
septate conidiophores, hyaline acicular conidia, multiseptate, from 2.5 to
4 µm wide and 50-200 µm long. |
| Host range: |
Propagative on Beta
vulgaris and most species of Beta. Reported on members of the
Chenopodiaceae and on Amaranthus. |
| Disease symptoms: |
Infected leaves and
petioles of B. vulgaris exhibit numerous circular leaf spots
that coalesce in severe cases causing complete leaf collapse. Dark specks
within a grey spot centre are characteristic for the disease. Older leaves
exhibit a greater number of lesions with larger spot diameter. During the
latter stage of severe epiphytotics, new leaf growth can be seen emerging
from the plant surrounded by prostrate, collapsed leaves. |
| Control: |
Fungicides in the
benzimidazole and triazole class as well as organotin derivatives and
strobilurins have successfully been used to control Cercospora leaf spot.
Elevated levels of tolerance in populations of C. beticola to
some of the chemicals registered for control has been documented. Partial
genetic resistance also is used to reduce leaf spot disease. |
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Micrographs
of C. beticola conidiophores and conidia. Scanning electron
micrographs in panels A and B illustrate the topology of these structures
within disease lesions. Accompanying these are light micrographs of
conidiophores (C) and conidia (D).
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