Web blotch

Lesions subcircular, 1-10 mm diameter, reddish brown to black on the upper leaf surface and lighter shades of brown on the lower. Distinct chlorotic halos develop early on the upper surface, but their presence and prominence are altered by host genotype and environmental factors. Similar halos may be found around M. berkeleyi lesions; therefore the halo is not a good diagnostic character. The lesions tend to be larger than those of M. berkeleyi and the dark stroma of the latter is absent. 
The conidia form on both leaf surfaces, the conidiophores being somewhat diffuse. The distribution of fruiting structures, randomly on the adaxial surface for M. arachidis and in circular rings on the abaxial surface for M. berkeleyi, are useful characters for distinguishing between the two leaf spots in the field. Severe attacks cause defoliation. Leaf spot disease symptoms are influenced by host genotype and environmental factors. For both diseases, small chlorotic spots appear on leaflets 10 days after infection. The spots then develop in about 5 days into mature, sporulating lesions. The two pathogens can be readily identified by the morphology of conidiophores and conidia. Examination of sections of diseased leaflets shows that M. berkeleyi produces haustoria within host cells, whereas M. arachidis does not.
In addition to causing leaf spots, the two pathogens also produce lesions on petioles, stems, and pegs. These are oval to elongate and have more distinct margins than the leaflet lesions. When disease attack is severe, the affected leaflets first become cholorotic, then necrotic, lesions often coalesce, and leaflets are shed.