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Disease Management

Fungicides

Large plantations are heavily reliant on chemical controls. Control programs are largely based on the protectant fungicides mancozeb, usually applied in water or in combination with oil, and chlorothalonil. Mancozeb is often combined or rotated with morpholine, demethylation inhibitors (DMI), or strobilurin (QoI) fungicides. Cholorothalonil is rotated but not combined with other fungicides. Resistance to benzimidazole, DMI, and strobilurin fungicides is widespread in many production areas. The fungicides are often applied by airplanes (Figure 16).

Figure 16

Biological Control

Research into developing biological control methods for black Sigatoka has been limited since highly effective and affordable chemical controls are widely available for commercial growers. While biological control methods are desirable primarily for environmental reasons, their successful implementation is likely to be difficult since black Sigatoka is a polycyclic disease and susceptible banana tissue is present year-round. Various epiphytic bacteria (including Pseudomonas, Bacillus, and Serratia spp.) have been tested for controlling M. fijiensis, but biological control research is still in its preliminary stages.

Resistant cultivars

Use of resistant cultivars is really the only practical means of black Sigatoka control for the small-scale and subsistence farmer since fungicides are generally unaffordable. Unfortunately, while some resistant cultivars of plantains and bananas are available, they are often unacceptable to local tastes. Developing acceptable resistant cultivars is an important priority at international research centers (Figure 17).

Figure 17

However, breeding for disease resistance is particularly difficult with banana plants. Commercial bananas are autotriploids (AAA), i.e., they have three copies of the chromosome set instead of the two copies that the wild diploid species possess. While this extra chromosome set imparts favorable commercial characteristics such as seedlessness (unlike diploid wild species, Figure 18), as well as larger plant and fruit size, this high level of sterility is a major obstacle to banana breeders. Furthermore, the generation time (from seed to seed) for bananas is a long three years.

Figure 18

Cultural management

Cultural management techniques such as wider plant spacing, better drainage of both water and air, better weed management, and removal of severely diseased leaves or portions of them from plants can also be used to obtain some measure of control. Simply removing infected leaves (deleafing) and placing them on the ground can significantly reduce the efficacy of ascospore discharge (Figure 19). The application of urea and other products onto infested plant debris on the ground can accelerate decomposition and thus reduce further the available spore inoculum.

Figure 19

Quarantine and Sanitation

Proper quarantine and sanitation measures may provide some protection against two common means of long distance inoculum dispersal—leaves and rhizomes. Contaminated banana leaves are often used to protect the fruit when transporting bananas by truck.

Quarantine measures are practiced in some areas and countries where M. fijiensis has not yet been established or is confined to certain areas (Figures 20 and 21).

Figure 20 Figure 21