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which cactus pear is cultivated, it is likely to spread effectively throughout the region. Presently, cactus pear cannot be cultivated in South Africa on a large scale without the interference of this cochineal, due to the insect's effective dispersal abilities. Figure 14.4 shows countries into which Dactylopius opuntiae has been introduced intentionally for the biological control of Opuntia weeds. These countries now serve as focal points for the further spread of the insect.

The movement of certain other cactus-feeding insects for biological control purposes also decreases their distance from new targets. Some of these insects have the potential to become serious pests on cultivated opuntias and may also threaten native Opuntia species. The recent dispersal of Cactoblastis cactorum within the West Indies and eventually to mainland Florida, either through inadvertent intro ductions by the nursery trade (Pemberton 1995) or through natural dispersal, has serious consequences for both the native Opuntia flora and cultivated plants in the United States and Mexico (Zimmermann and Perez-Sandi y Cuen 1999). Figure 14.5 shows the history of the introduction and spread of C. cactorum from Argentina since 1925. The many interceptions at the main entry points into the southeastern United States of cactus plants containing endophagous C. cactorum larvae and belonging to the cactus nursery trade demonstrates how easily the insect's entry into the country might pass unobserved (Pemberton 1995). A lively cactus trade also exists between Europe and many American countries, and the arrival of the insect into Mediterranean countries is therefore almost inevitable.

In the state of Pernambuco, Brazil, cultivated Opuntia species are important as a source of fodder, and about

Calvin Hobbes Thanksgiving
Figure 14.4. Countries where Dactylopius opuntiae has been intentionally released for the biological control of Opuntia weeds.

400,000 ha are under cultivation (Barbera 1995). This can provide an easy target for both C. cactorum from the south and Dactylopius opuntiae from the north. Increased cultivation of cactus pear between Paraguay and Pernambuco will eventually form a host chain that might allow C. cac-torum to spread naturally to these areas. The Andes form a natural barrier between the Opuntia species infested with

C. cactorum in Argentina and the pest-free cultivations in Chile but, with increasing cultivation of O. ficus-indica in some Andean valleys in Argentina, these distances have decreased considerably. Within Africa, both C. cactorum and

D. opuntiae may disperse northward from Zimbabwe via Kenya to Ethiopia because the cultivation of O. ficus-indica in the arid zones between these two countries is being encouraged.

Little is known of the dispersal behavior of the other major cactus pear pests in Mexico. However, with the increasing cactus trade and the large number of amateur cactus collectors moving cacti between countries and continents, the inadvertent dispersal of new insect pests is inevitable.

Diseases of Cultivated Cacti

The most important cactus pear diseases can be grouped according to their pathogenic agents (Table 14.2). The bi-otic diseases are caused by bacteria, yeasts, fungi, phyto-plasmas, viruses, and some not-so-well-defined agents called phytoplasma/virus-like organisms. Some diseases and disease-like symptoms on cactus pear can also be caused by abiotic conditions, e.g., environmental stress (such as hailstorms), genetic anomalies, incorrect pesticide application, and physiological disorders. Biotic diseases of cactus pear are present in all the growing areas. They are influenced by the presence of the pathogen as well as climatic conditions. For example, bacterial diseases are more prevalent in Chile, Argentina, Mexico, and Italy than in Peru and North Africa because the elevated temperatures and low humidity in the latter two regions do not favor this type of disease. Although fungal diseases are also influenced by environmental conditions, some have not been reported in certain countries, probably because the pathogen is not present in that region. For example, Alternaria golden spot is present and causes severe damage in Mexico, Italy, and South Africa but not in other cactus-pear growing countries; Aecidium spp. provokes major disease only in Peru; Cercospora spp. is prevalent in Peru and Bolivia; Phoma sorghina causes damage only in Argentina; and Dothiorella ribis, the causal agent of a cactus-pear gum cancer, has been found only on the small island of Linosa (Italy). Few systematic studies have focused on cactus pear viral diseases, even though various reports exist (Chessin

The evolution of cactus diseases is very rapid, as the biochemical characteristics of the cell juices adapt well to the various biotic agents' requirements. This extremely important fact should highlight the urgency of preventing diffusion of diseases that can rapidly become epidemic. Strict control must be performed on propagation material. Prevention is the best approach to guarantee successful cultivation of cacti, and propagation material and fruit should not be imported from areas where specific diseases are present.

Figure 14.5. Intentional and unintentional introduction of Cactoblastis cactorum. Establishment in Kenya and Pakistan is not confirmed.

Unintentional:

Cuba, 1991, Haiti, Dominican Republic, Puerto Rico, Bahamas, Florida, 1989,

Unintentional:

Cuba, 1991, Haiti, Dominican Republic, Puerto Rico, Bahamas, Florida, 1989,

Figure 14.5. Intentional and unintentional introduction of Cactoblastis cactorum. Establishment in Kenya and Pakistan is not confirmed.

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