Alan
McNab
Professor of Plant Pathology |
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| CARROT
DISEASES |
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Leaf Spots are caused
by the fungi Cercospora carotae and Alternaria dauci and by the bacterium
Xanthomanas carotae.
Cercospora leaf spots are brown to gray and are more prevalent
on young foliage than on old foliage. Alternaria leaf spots generally
are dark brown to black and are more prevalent on older foliage than
on young foliage. Xanthomonas leaf spots are similar to Alternaria
leaf spots; the two cannot be distinguished without microscopig examination.
Yellow margins may be present around spots caused by each organism.
The fungi and bacteria are seed- and soil-borne. Cercospora and spores
are spread by wind. Xanthomonas cells and also the fungus spores are
spread in splashing and running water, wind-blown soil, and on implements.
Cercospora and Alternaria can infect leaves only when they are wet.
Xanthamonas can infect leaves when the relative humidity is high (90
to 100%) for a 2- to 3-day period. |
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Root Knot
infected carrots may have forked roots and irregular round galls and
spindle-shaped enlargements on the tap and side roots. These symptoms
are caused by the same kind of root knot nematode (microscopic worm)
that causes root galling on tomatoes, cucurbits, lettuce, and other
vegetable crops. Several species of the root knot nematode (Meloidogyne
spp.) may be involved. Only the northern species (M. hapla) is thought
to overwinter in northern areas exposed to cold winter temperatures.
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Aster Yellows
of carrots, caused by a mycoplasma, is characterized by production
of yellowish dwarfed leaves, usually arranged in a tight rosette.
Older leaves may develop reddish margins. The root at the crown bulges
up into a cone and hair-like roots develop on the taproot. The mycoplasma
overwinters in many perennial weeds and is spread by leafhoppers. |
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Storage Rots
of carrots are caused by fungi and bacteria. In a New York study,
crater rot caused by the fungus Rhizoctonia carotae caused the most
rot during the first 4 months of storage. During the latter half of
the storage period, most of the rots that developed were gray mold
caused by Botrytis cinerea crown rot caused by Rhizoctonia sp., and
black rot caused by Stemphylium radicinum. Other storage rots include
bacterial soft rot caused by Erwinia spp. of bacteria, cotton (watery)
soft rot caused by Sclerotinia sclerotiorum, wooly soft rot caused
by Rhizopus spp., Fusarium dry rot caused by Fusarium roseum, and
licorice rot caused by Centrospora acerina. The New York study revealed
that the single most important factor affecting losses from storage
rots was speed of cooling after harvest.
The more rapidly carrots were colled to the 31 or 32 ° F storage
temperature, the less rot that developed. Soil moisture conditions
and mechanical injury also affected incidence of rot. More rot developed
in stored carrots from poorly drained than from well-drained soil.
Carrots mechanically injured during harvest and during preparation
for storage were more likely to develop rot. High relative humidity,
a requirement for long-term storage, did not result in excessive rot
in carrots cooled promptly after harvest. Storage crates were not
an important source of disease organisms. |
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