An overview of physiological specialization of coffee leaf rust: New designation of pathotypes
Laércio Zambolim and Eveline Teixeira Caixeta
Coffee leaf rust is the most important disease thorought the world. The disease caused by a biotrofic fungi Hemileia vastatrix Berk & Br. has been a serious threat for the sustainability of all coffee growing areas of the world. It caused the destruction of the coffee crop in Sri Lanka (Ceylon). Today, coffee leaf rust causes losses from 35 – 50 % on average in Brazil and more tham 50% of the entire Central American coffee park. Coffee rust probably originates in southwestern Ethiopia and was identified in 1860. Then the disease spreaded across the Indian and Pacific Oceans (Philippines, Madagascar and Java). In the 1950s and 1960s, rust advanced across West Africa. In January 1970, the disease was identified in Brazil. In the 1970s-90s, leaf rust advanced to South America, Central America, Caribean and Mexico. Currently, more than 50 H. vastatrix physiological races have been identified by using coffee differentials clones of CIFC in the world. The most comum and widespread H. vastatrix race II is presente in all over the world. The CIFC diferential clones has nine resistance genes (SH, SH2, SH3, SH4, SH6, SH7, SH8 and SH9) alone or in combination. Races are identified by the differential interaction of genes of the host and of the pathogen. Molecular biology technique, found two different genes in HdT 832/1 and HdT 832/2 not yet identified in differential clones (SH1-9), named as SH10 and SH11. In the present review, we are proposing the name pathotype to refer the isolates of H. vastatrix that could not be differentiated into races, according to the CIFC system. Several pathotypes was found in several countries. It became evident that race nomenclature based on a set of differentials of CIFC was not enough to characterize complex isolates from HdT derivatives into races. The resistance of the differential clones HdT 832/1 (SH 6,7,8,9,? - v6,7,8,9,?) and HdT832/2 (SH 6,7,8,9,? - v6,7,8,9,?) was only supplanted in India by isolates colleted in the field; but their resistance was not supplanted in any other part of the world. On the other hand all the progenies derived from HdT 832/1 and HdT 832/2 cossed with arabica coffee lost the complete resistance to H. vastatrix in the field in all the coffee growing areas of the world. The only gene that complete resistance was not suplanted yet is SH3, in the dominant form, from C. liberica. This finding is very important due to the fact that cultivars containing the gene SH3 may be attacked only by the race that has the gene v3, which is absent in most of the coffee growing countries. For these reasons coffee breders must consider in therir breeding programs cross HdT 832/1 and 832/2 (SH 6,7,8,9) with S 288-23 (SH3). Since there is no complete resistance to the disease all over the world growers has to rely on the chemical control of the disease. Finaly this overview will discuss designation of races and pathotypes according CIFIC system and Flor gene for gene theory. By the CIFC system the pathogen is recessive and virulent (v1-9) and the host plants are dominant and resistant (SH1-9) and the interaction is susceptible (S). In our proposed system, the pathogen is avirulent (Avr1-9) and the host plant is also dominant (SH1-9) and the interaction is resistant (R) characterizing the hypersensitive reaction.