Mycosphaerella leaf spot diseases of bananas - CBS

Session 1
D.R. Jones
Movement between continents and between isolated countries, such as those found
in the Pacific, is probably the result of the transfer of diseased material by humans.
The appearance of the disease in Australia in the mid-1920s may have been due
to the same movements of planting material from the South Pacific that are believed
to have introduced bunchy top disease (Magee, 1953). Another possibility is that
diseased banana leaves may have been used as packing material for goods shipped
into Australia from the South Pacific.
Sigatoka disease may have been introduced to East Africa in the late 1930s by
the movement of planting material/diseased leaves from Asia. During the colonial
period, there was much migration to East Africa from India and settlers and/or British
administrators undoubtedly transported infected suckers of their favourite cultivars
from one location to another.
The appearance of Sigatoka disease in European colonies in the eastern Caribbean
region in the mid-1930s could also have been due to colonial-inspired movements
of Musa germplasm from Asia. The almost simultaneous discovery of the disease at
several locations may have been due to multiple introductions of the pathogen or
to rapid spread from one point after an undetected build-up of inoculum. Prevailing
landward winds off the Caribbean Sea would have carried spores of M. musicola
across the north of South America and into Central America.
Today, Sigatoka disease is regarded as having a worldwide distribution, although
it has not been recorded in the Canary Islands, Egypt or Israel (Meredith, 1970). The
dry summer climates of these countries may make the local environment unsuitable
for disease establishment.
Impact
The most efficient leaves for photosynthesis on a vegetatively growing banana are
the second to fifth counting down from the top of the plant. Therefore, if optimal
assimilation potential of the plant is to be maintained, it is important that leaves
2-5 are free of excessive shade, severe leaf tearing and disease. In a vigorous plant
growing in the tropics, this critical leaf area is renewed monthly (Robinson, 1996)
and the pathogen does not cause enough damage to have an appreciable effect on
growth (Leach, 1946). Damage comes after bunch emergence when leaf production
ceases and leaf tissue cannot be renewed. The greater the damage on remaining leaves
and the earlier it occurs after shooting, the greater the effects on yield. Sigatoka
disease also affects the physiology of developing fruits causing premature ripening
(Meredith, 1970). This occurs in the field if the plant is severely diseased or in transit
to markets if moderately affected. For these reasons it was important to control
Sigatoka disease in commercial plantations.
The economic impact of Sigatoka disease has been twofold. First, there was the
direct effect on production when the disease first became established and control
measures were being developed. Between 1937 and 1941, production in Mexico was
halved as a direct result of Sigatoka disease. In Honduras, production declined to
less than one third of the pre-disease level (Meredith, 1970). These were enormous
losses. Secondly, there were the on-going costs associated with leaf spot control once
effective measures were developed and adopted. The struggle against Sigatoka disease
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