Agriculture Reference
In-Depth Information
Currah (2002) gives lists of onion cultivars grown in Africa, tropical and sub-
tropical Asia, the Americas and the Caribbean.
19.2 ONION DISEASES
Onions are attacked by viruses, phytoplasmas, bacteria and fungi (Schwartz and
Mohan, 1995). The case histories given in this chapter relate to fungal pathogens of
onions and illustrate epidemiological concepts. Johnston and Booth (1983) defined
epidemiology as a 'Study of the factors affecting outbreaks of disease and spread of
infection' and this definition is followed here. The epidemiological factors, given
below, that influence infection are being used by scientists to produce predictive
models for the onset of diseases, particularly those caused by foliar pathogens. Some
of these models will be considered in this chapter.
Climatic factors, particularly temperature, rainfall, relative humidity (RH),
duration of leaf wetness, crop phenology and nutritional status are selective of the
range of fungal pathogens which may occur in temperate (cool dry and cool moist
conditions) and tropical (hot dry and hot moist conditions) countries. Light influences
the sporulation of fungi and the spore form (i.e
.
ascospores, pycnidiospores, and
conidia) and dictates their method of distribution. Inoculum source (seed, soil, debris,
etc.) are foci for disease transmission; disease progress depends on inoculum
concentration and on the operation (or not) of some of the factors given above.
19.3 CASE HISTORIES: SEEDBORNE DISEASES
19.3.1 Constraints affecting seeds as infection sources
(a) Temperature implications
Temperatures that are optimal for the growth of pathogens of onion in culture (Table
19.2) may be broadly indicative of the climatic temperatures that will favour their
development as disease agents in crops.
The microflora of onion seeds comprises a mixture of organisms - bacteria and
fungi in particular; some of these are superficial and others are more deeply situated
in the tissues of the seeds. The respective locations of organisms on or within seed
tissues is a direct result of the available sources of inoculum and of the environmental
conditions that obtained when the seed crop was growing and maturing.
The mycofloral composition of seeds is largely determined by the temperatures
and moisture conditions (rainfall, duration of leaf wetness and RH) that occurred
during seed crop production. Some of these fungi are pathogens of onions; their ability
to transmit and cause disease depends on their interactions with other seedborne and
soilborne organisms within the temperature profiles of their respective environments.
For example,
Botrytis aclada
(
Botrytis allii
) grows optimally in culture at 21.5
o
C
and is inhibited by temperatures of 30
o
C (Fig. 19.1). Onion seeds produced under
temperate conditions may be infected with
Botrytis aclada
. Infected seeds transmit
the fungus to onions grown in cool moist climates, such as that of the UK, causing
