Abstract Fungal pathogenicity in plants requires a conserved mitogen-activated protein kinase (MAPK) cascade homologous to the yeast filamentous growth pathway. How this signaling cascade is activated during infection remains poorly understood. In the soil-borne vascular wilt fungus Fusarium oxysporum, the orthologous MAPK Fmk1 (Fusarium MAPK1) is essential for root penetration and pathogenicity in tomato (Solanum lycopersicum) plants. Here, we show that Msb2, a highly glycosylated transmembrane protein, is required for surface-induced phosphorylation of Fmk1 and contributes to a subset of Fmk1-regulated functions related to invasive growth and virulence. Mutants lacking Msb2 share characteristic phenotypes with the Δfmk1 mutant, including defects in cellophane invasion, penetration of the root surface, and induction of vascular wilt symptoms in tomato plants. In contrast with Δfmk1, Δmsb2 mutants were hypersensitive to cell wall targeting compounds, a phenotype that was exacerbated in a Δmsb2 Δfmk1 double mutant. These results suggest that the membrane mucin Msb2 promotes invasive growth and plant infection upstream of Fmk1 while contributing to cell integrity through a distinct pathway.