Plant diseases with trypanosomatid aetiology in Latin America and the Caribbean

The existence of trypanosomatids in plants has been known since 1909. However, the protozoology field was ignored for a long time by plant pathologists for several reasons: first because those organisms virtually only concerned non-cultivated latex plants; they were not associated with a characterized disease syndrome; they had never been in vitro cultured. The first phytopathological problem revealed by Stahel in 1931 was coffee tree phloem necrosis in Surinam. But it was only in the 1970s, with the discovery of the specific association of trypanosomatids with wilt diseases of cultivated palms in Latin America that a real interest was taken in these organisms. Hartrot of coconut is an inappropriate name for the trypanosomatid disease of Cocos nucifera. In fact, the hartrot syndrome is the same as the Lethal Yellowing disease caused by phytoplasmas. The disease starts with yellowing of the lower leaves, which gradually spreads to the younger leaves. It is only at the final stage that the apical region rots. Marchitez sorpresiva of Elaeis guineensis is a sudden wilt with the same symptom evolution as hartrot, but yellowing is replaced by browning. In less than 5 weeks the whole leaf mass is dried up and the palm dies. In both cases, hartrot and Marchitez sorpresiva, the trypanosomatids are transmitted by Hemiptera of the Pentatomidae family. Several species of the genus Lincus can transmit pathogenic trypanosomatids into the sieve tubes of palms. Coffee phloem necrosis was a serious problem in Surinam in the 1930s, affecting varieties cultivated at that time (Coffea liberica, C. excelsa, C. arabica, etc.). That disease did not attract much attention as it occurred in a small coffee growing area and on species of low economic importance. It was still present in Surinam in the 1960s, and the last report of trypanosomatids in a diseased coffee tree was in 1977 in Trinidad. The most recent trypanosomatid wilt concerns red ginger (Alpinia purpurata, Zingiberaceae) in Grenada, in the Caribbean. The disease, which was reported for the first time in 1989, starts with chlorosis, turning into yellowing and marginal necrosis, and complete browning of several leaves. The juvenile stem and inflorescence primordium aborts. Seventy-five per cent of a field can be affected after 2-3 years leading to the crop being abandoned. The trypanosomatid disease can be diagnosed on fresh sap, using light microscopy at a magnification of 40 x 10, after minimum training. However, the result depends on the stage of the disease, the choice of sample, and the quality of the microscope. Phase contrast greatly facilitates observation. Confirmation of Trypanosomatidae affiliation is obtained with Giemsa staining of smears on a microscope slide. Although only one genus name-Phytomonas-as been proposed as yet for all trypanosomatids found in plants, they can differ from one another in terms of biological properties and their effects. It even happens that "insect trypanosomatids" (Leptomonas, Herpetomonas, Crithidia) can be isolated from fruits, flowers or seeds. Recent serological and molecular marker studies have clearly showed that Phytomonas is an artificial taxon and does not reflect the extreme heterogeneity of the trypanosomatids isolated from plants. Phloem restricted trypanosomatids, specifically associated with wilt diseases of cultivated crops in Latin America and the Caribbean form a clearly characterized monophyletic group. The term "phloemicola" has been proposed to name them. In Latin America, other oil palm or coconut diseases or disorders are referred to in a recurrent manner as "Marchitez" causing confusion between different diseases. A basic and elementary real description of the syndrome should avoid any confusion. (Texte intégral