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Sooty blotch occurs in wet or humid conditions when the fruit remain moist for extended
periods. It is commonly found on fruit in the centres of trees and, particularly if there
are tall weeds, on the lower branches.
Pruning (to open up and allow aeration of the centres of trees) and skirting (to lift branches
above the grass) will help to reduce the incidence of sooty blotch. The routine fungicide
spray programmes early in the season for melanose and verrucosis, and later for brown
rot (Phytophthora), will also protect against sooty blotch.
Green and blue mould
Green mould (Figure. 6.2.7), caused by the fungus Penicillium digitatum, is the most
destructive post-harvest disease of citrus in New Zealand. Not only can a large proportion
of the fruit become rotted, but also fruit lying adjacent to or below rotted fruit in the
boxes can become contaminated and disfigured by masses of powdery spores.
The first symptoms of green mould appear two to three days after infection as a clear or
slightly discoloured soft, water-soaked spot around a point of injury. The spot expands
rapidly (up to 30-40 mm in diameter within 24 hours at temperatures of 21-25ºC). A
white fungal growth (the mycelium) appears on the surface of the rot and continues
to spread as the spot enlarges, usually remaining about 10 mm behind the expanding
edge of the rot. The characteristic masses of olive-green spores are produced first at
the centre with the sporulating area extending out over the rot. The whole fruit may be
rotted within two to three days and become covered by a dry, dusty, olive-green spore
mass. In some cases, practically the whole fruit is affected by the clear rot before surface
fungal growth and sporulation occurs. This variation of green mould development is
often mistaken for sour rot caused by Geotrichum candidum. If the relative humidity
remains low, e.g., in packing sheds, the rotted fruit may form a wrinkled, dry mummy.
Under moist conditions, as on the orchard floor, yeasts and other fungi also colonise the
fruit and it rapidly decomposes.
The fungus is found almost everywhere in orchard soil and debris, and is common in dust
and debris in and around packing sheds. The fungus sporulates prolifically and the spores
are readily spread by wind and rain splash. Green mould generally requires physical
injury to the skin for infection. Spores present on the surface of the fruit germinate in
the wound. Even the slightest injuries can lead to infections; most of these probably
originate in the field, either from natural injuries or during harvest. The incidence of rots
generally increases as the season progresses, with large numbers of infected windfall
fruit producing high levels of inoculum in the orchard and heavy dews or wet harvesting
conditions providing ideal conditions for infection. Damage inflicted during subsequent
packing provides further opportunity for infection. Because, in general, injury is necessary
for infection, there is usually little spread from rotted to sound fruit during transit.
The fungus can grow over a wide range of temperatures, from around 3ºC to above
35ºC. The maximum rate of growth occurs from 25-28ºC. At normal citrus coolstore
temperatures of 3-5ºC, infections establish and spread slowly. Removal to warmer
conditions then leads to the rapid collapse of those fruit.
Blue mould (Figure. 6.2.8), caused by the related fungus Penicillium italicum, is less
common than green mould but can cause considerable losses in cool-store fruit.
The early symptoms are similar to those of green mould, with a clear water-soaked
spot expanding from the point of infection. Generally the rot expands more slowly than
green mould and may not cover the whole fruit. The most distinguishing feature is the
distinctive blue colour of the sporulating area, surrounded by a relatively narrow band
of white, non-sporulating mycelium.
Affected fruit fall readily from the tree, and in high humidity, white fungal growth may
become evident on the fruit surface.
Phytophthora grows and reproduces on both living and dead tissue in almost all New
Zealand soils. Because the fungus requires water for spore production and infection, root
and trunk rots occur most often in sites with poor drainage and frequent waterlogging.
Fruit infections are initiated during wet conditions when spores are splashed from the
soil on to fruit that are touching, or near, the ground. If wet weather persists, spores
produced on those fruit are splashed or blown on to fruit higher on the tree.
The disease attacks fruit of all cultivars and can be particularly serious on lemons and
New Zealand grapefruit.
Brown rot can usually be controlled by a combination of cultural and chemical means.
The ground should be kept free of ponded water by surface drains or contouring, and
weeds and low branches should be removed to promote air circulation and to reduce
contact between soil and leaves and fruit. The orchard should be managed to encourage
rapid drying of fruit and foliage after rain. Shelter belts should be topped to avoid excess
shading, and thinned to ensure air movement through the orchard. Overgrown trees
should be pruned to encourage air movement through the canopy.
Fungicide sprays applied to the soil and the lower part of the tree before anticipated wet
weather help to prevent brown rot. Copper fungicides (copper oxychloride and the cupric
hydroxide range), Dithane M45 and Captan have approved label claims for the control of
Phytophthora in citrus. Applications are normally made in autumn to provide protection
into the harvest season. In very wet conditions, or in orchards with a previous history of
heavy infection, a second application may be necessary in July for late harvested fruit.
Sooty mould
Sooty mould is a black, superficial fungal growth on the fruit, leaves, and stems of citrus
trees that are infested with sap-sucking insects, such as scale insects, aphids, mealybugs,
and whitefly. These insects excrete a sugary substance (honeydew) which provides the
food base on which the fungus grows. A range of different fungi can be associated
with sooty mould, the most common one in New Zealand being Capnodium salicinum.
Although sooty mould does not penetrate the host tissues, in severe cases it can affect
tree performance by intercepting light and reducing photosynthesis. Sooty mould on the
fruit makes it unsuitable for sale. Washing or brushing may remove most of the mould
but enough usually remains to reduce the value of the fruit.
In general, problems with sooty mould indicate high populations of sap sucking insects,
which will need to be controlled. Routine spray programmes for soft scale, whitefly, etc.,
will generally eliminate problems with sooty mould. Oil sprays will have a dual effect
of controlling some of the pests that produce honeydew as well as loosening the mould
deposits, assisting its removal by wind and rain. Routine copper spraying also helps to
reduce the incidence of sooty mould.
Sooty blotch
Sooty blotch is a light-brown shading of the surface of fruit, in patches, or in extreme
cases covering most of the surface. The blotches are caused by a single layer of dark-
brown fungal threads spreading over the surface. Although the fungus does not penetrate
the fruit, it is embedded in the cuticle and cannot be removed by washing. Several fungi
are known to cause sooty blotch, the most common being Gloeodes pomigena.