Agriculture Reference
In-Depth Information
Figure 19.25
Oedema on
Pelargonium
leaf. Note the
raised, corky spots
This effect is most often seen in greenhouses, where
overwatering, lack of heating or poor ventilation can
affect plant growth. Oedema occurs when the roots'
ability to supply water exceeds the leaves' ability to
release the water by transpiration. Conditions favouring
oedema occur most commonly in late winter and early
spring, especially during extended periods of cool,
cloudy weather. Warm, moist soil or compost occurring
alongside cool, moist air brings on the condition
most severely. The symptoms are commonly seen in
unheated greenhouses. The problem can be greatly
reduced by glasshouse heating and automatic venting.
Figure 19.26
Rose balling. Note the papery, dead
petals on the outside of the rose fl ower
may grow on damaged buds and cause stem die-back.
In such circumstances, shoots should be pruned back
to the healthy part of the stem.
Soil pH
Garden species of plants have originally developed
in different habitats and different parts of the world
(see Chapter 2). Most species grow best at pH 6 and
7 but may have a tolerance to more acid or more
alkaline conditions (see p. 175). Outside their limits of
tolerance, roots may be subjected to two problems:
an inability to absorb a balanced level of nutrients
and an inappropriate balance of soil micro-organisms.
Above-ground, growth is stunted and main veins may
become blackened (see Figure 19.27).
Rose balling
This is a condition in which the outer petals of the
rose flower die and become stiff. The inner petals are
thus prevented from emerging to produce a normal
bloom. The flower bud remains globular in shape
(see Figure 19.26). Rose balling is caused by damp,
cool conditions, and is most often seen in places that
are shaded for part of the day. The wet outer petals
are not able to dry out before being scorched by the
direct sun. The outer petals are turned first into a soft
dead tissue that quickly dries to form a quite tough
layer, preventing any petals inside from emerging.
Rose cultivars that have many slender petals ('double-
petalled') are particularly prone to this problem. The
withered blooms eventually drop off the plant.
Affected buds should be cut off. Care should be taken
not to increase the problem by splashing water onto
buds when watering roses. Grey mould (see p. 258)
Lime-induced chlorosis
This is a quite common condition that illustrates how
an unsuitably high pH may affect nutrient uptake
and plant growth (see Chapter 14, p. 171). Here,
the leaves of the plant show an unhealthy yellow
appearance, especially between the veins (interveinal
chlorosis, see Figure 19.28). Young leaves are
particularly affected. Raspberries, top fruit,
Hydrangea,
Rhododendron
,
Camellias
and
Skimmia
are examples
of plants prone to this problem.
