Evolution of citrus canker (Xanthomonas axonopodis pv citri) in lemon with and without summer copper sprays on new leaves before bloom

Abstract

Citrus canker, caused by Xanthomonas axonopodis pv citri is one of the main diseases that affect the citrus production. The effect of copper applications on summer and fall leaf flushes in lemon on citrus canker control was studied. This work was done on lemon (Citrus limon) cv Lisbon budded on P. Trifoliata at de end of year 2003 and during all 2004. Three leaf flushes were marked at onset on the four cardinal points of each tree. The length of shoots, leafnumber and number of diseased leaves and canker severity was assessed. The rating of foliar disease severity used included four levels from 0: healthy, 1: between 1 and 20 lesions per leaf, 2: between 21 and 50 lesions and 3: more than 50 lesions per leaf. The temperature and the relative moisture of the air were recorded as well as the rainfall and crop management. Treatments applied were: I) six copper sprays from August to December and II) ten sprays from August to April. The statistical design was complete random blocks arranged according to the initial distribution of the diseased plants. Non lineal monomolecular models (Campbell y Madden, 1990) were adjusted for disease incidence from the beginning to 160 days later and their parameters were compared. A binomial distribution was assumed for the number of diseased leaves. The average severity index was adjusted to a lineal model of repeated measurements in time. The leaf flushes at the South part of the tree got larger amount of disease incidence than the other cardinal points due to the prevalent wind direction (P< 0,10). Canker incidence in plots with copper applications in summer (treatment II) was 8 to 13 % lower than the plots in treatment I (sprays from August to December) (P<0,0001). It is concluded that it is convenient to do copper applications in summer to reduce canker inoculum pressure at flowering time.