Agriculture Reference
In-Depth Information
Nematode effects
On light soils damage by the endoparasitic nematode
Pratylenchus penetrans
(Cobb) may cause patchy distribution of poor growth over a field, with nema-
todes being visible in the young roots. Root lesions develop on the unsuberized
roots. When these coalesce rootlets, or even complete root systems, may be de-
stroyed. Infected roots are often discoloured and stunted. Many plant species
includinggrassescanbeattackedbynematodes,sotheproblemisnotrestricted
to old apple land. Use of nematicides, e.g. D-D and methyl-isothiocyanate, as
a preplanting treatment can lead to major increases in yield, and marigold
(
Tagetes patula
) can be used as a preplant cover crop to reduce nematode popu-
lations (Mai
et al.
,
). Nematodes can, however, be transferred from infected
nurseries to orchards when the trees are planted there (Hoestra,
). Nema-
todes are not a major cause of replant problems of apple trees in the UK (Way
and Pitcher,
), in most Dutch apple growing areas (Hoestra,
)orin
Washington State (Willet
et al.
,
), although they are a significant problem
in New York State (Pruyne
et al.
,
).
Replant disease
In general the most severe check to growth of young apple trees planted on the
sites of old apple orchards is that caused by apple replant disease. This is not
due to nematodes, residual toxins, or induced nutrient deficiencies (Hoestra,
;Savory,
,
). Pot tests of responses to soil sterilization showed
% of soils in Dutch fruit-growing regions to have replant disease, half being
seriously infected (Hoestra,
% of orchard soils showed
economically significant responses to fumigation following adjustment of their
P content (Sewell
et al.
,
). In England
).
The main field symptoms are poor growth of both shoots and roots. Leaves
tend to be small and internodes short. Seedlings planted into replant soil
develop root lesions within a few days. Root hairs are reduced in size and
number, especially when in close contact with the soil. The primary cortex
and epidermis may rot away, leaving a thin, light-coloured stele covered by
remnants of the cortical layer. No abnormalities are found in the stele until
an advanced stage of decay is reached. If roots in 'replant' soil are allowed to
grow into sterilized soil they develop into healthy root systems with side roots
bearing many root hairs, i.e. the replant condition is relatively immobile in the
soil. The response of orchard trees to pre-planting fumigation is a function of
the soil volume fumigated ( Jackson,
).
Although the poor growth of apples after apples had long been recognized,
interest in the effect of the previous crop was enhanced following some
