Agriculture Reference
In-Depth Information
Fig 1.8 Fungi are major causes of disease in horticulture crops. They attack every organ of the plant including leaves, fruit and roots. Left:
blueberry rust (
Pucciniastrum vaccinii
); centre: stem-end rot in mango (
Lasiodiplodia theobromae
); right: Phytophthora in avocado roots
(
Phytophthora cinnamomi
).
Fungi
Fungi are mostly filamentous organisms that lack the green
pigment chlorophyll and must obtain energy from the
material on which they grow. Most fungi are saprobes,
living entirely on dead or decaying organic matter. Fungi
are the most important and most common cause of plant
disease, with about 23 000 species known to infect plants,
although it has been estimated that the actual number may
be as high as 270 000 species. Some fungal pathogens can
survive only by growing in their living host plants; these
are termed obligate parasites or biotrophs. Examples
include the rusts, smuts and powdery mildews. The
majority of fungal pathogens are non-obligate or facultative
parasites requiring a living host plant for only part of their
life cycle. Fungi consist of individual living filaments called
hyphae, which collectively form mycelium. As in other
organisms, reproduction is an essential part of the life cycle
of a fungus. Most fungi have the ability to reproduce both
sexually or asexually. Usually, the asexual (imperfect,
anamorphic) stage is the active pathogen and the sexual
(perfect, teleomorphic) stage may occur only rarely. The
sexual stage helps the fungus survive adverse, often
seasonal conditions, and provides genetic diversity for the
organism. The basic reproductive unit of fungi is the spore,
which germinates to produce hyphae. Spores may result
from both asexual and sexual reproduction and often a
single fungal species may produce several different types of
spores. The sexual stage of some fungi is unknown, or may
not even exist, and so only the asexual stage is known.
Formerly called
Fungi Imperfecti
, most are actually
ascomycetes or basidiomycetes.
Spores develop in special structures called fruiting
bodies, which provide some protection against desiccation
and ultraviolet radiation. Often produced in enormous
numbers, spores disperse by wind currents, rain, running
water or insects. Thick-walled spores resistant to adverse
conditions allow fungi to survive for long periods in the
soil and on both living and dead plants.
Fungal mycelium may also form small, hard structures
called sclerotia, which are important survival structures
for many fungi.
Table 1.1 Some physiological disorders of perennial fruit crops
CROP
DISORDER
CAUSE
Apple
Jonathon spot
Bitter pit
Tissue senescence after removal of fruit from cool storage
Associated with low levels of calcium in the fruit
Complex relationship between climate, nutrition and tree vigour
Pear
Superficial scald
Oxidation of organic compounds causing collapse of skin cells and tissue browning
Avocado
Cocktails (cukes)
Ringneck of fruit
Poor pollination and premature seed abortion
Water stress
Papaya
Lumpy fruit
Boron deficiency
Grape
Sulfur dioxide injury
Absorption of excess quantities of sulfur dioxide during treatment for grey mould
Citrus
Oleocellosis
Damage to rind tissue by oil released from damaged oil glands of fruit
