Potassium phosphite (Canon®) was evaluated as a means to control Alternaria brown spot, caused by Alternaria alternata pv. citri, which inflicts severe damage on susceptible citrus cultivars worldwide. Canon®, at concentrations up to 400 gMg/ml, did not inhibit germination of conidia of A. alternata pv. citri, but it did inhibit mycelium growth, slightly at 3.12 gMg/ml and completely at 50 gMg/ml. These concentrations are much higher than those of Sygnum® that gave comparable inhibition. Foliar protection assays revealed that prophylactic spray with Canon® decreased lesion development on leaves, even though it had a negligible effect on spore germination and subsequent mycelium development. Laboratory inoculation assays revealed that fruits and leaves collected after treatment with Canon® were often unprotected, whereas recommended fungicides were effective. In eight experiments conducted in 2001 to 2003 in groves of Minneola or Nova (Yanuv) tangelos and murcott, the effects of fungicides were evaluated according to the number of lesions per fruit. In all the experiments, Canon® significantly increased the proportion of export quality class I fruits (up to five small lesions per fruit), compared with untreated controls, and its performance did not differ from that of the commercial treatment with recommended fungicides on alternating schedules. The proportion of exportable fruits was often 50–100% greater than that in the untreated control. Moreover, the proportion of severely infected fruits (category 3 infections) was sometimes more than 10 times as high in the control than in the Canon® treatment.