Aphids, funguses, and bacteria are common diseases that attack the foxglove plant. When any of these diseases appear, you should address them immediately to rid the plant of the infestation and to prevent it from spreading to other plants. Early detection is the best weapon to use in the prevention of disease to the foxglove plant. Aside from that, using organic pesticides and sprays may also help prevent the growth and spread of common diseases to which the foxglove plant is vulnerable.
Aphids are tiny insects that attach themselves to the leaves and stems of the foxglove plant. When they do they eat the plant and cause it to become discolored. Over time as the infestation of aphids gets worse the plant will die. Nearby plants will also become infected with aphids after the foxglove plant dies.
Aphids should be treated immediately with insecticides to keep them from spreading to the healthy portion of the foxglove plant. You can also prune the plant, cutting away the affected leaves. Aphid infestation should be monitored on a constant basis.
Plants grown indoors are especially subject to some airborne or transmitted fungus. Fungus that affects a foxglove plant will cause the flowering portion to either wither, become discolored or fall off. Fungus should also be treated immediately with a plant fungicide or other organic treatment designed to keep the fungus from spreading. You can also prune affected leaves in the same manner that you pruned them for aphids to rid the plant of the fungal growth.
Bacteria can affect the quality of the plant's soil and appearance, causing it to become black or discolored and eventually die. Bacteria typically finds its way into the soil of the foxglove, so maintaining a proper pH balance is important to prevent the growth of bacterial spores. Ridding the garden of the parts of the plant that is affected will solve any immediate signs of bacteria in the soil. Add a soil treatment designed to rid the soil of bacteria.