Barberry bushes carry several diseases, but the disease most farmers are worried about is stem disease. Stem disease affects grain crops, but is not transferred from the stubs left after cutting the grain. The stem disease transfers to nearby barberry bushes, then infects the next year's crops. Farmers remove barberry bushes near grain crops, so that the disease cannot spread to their crops every year.
Stem rust shows up on barberry bushes as reddish brown pustules. The pustules are found on several parts of the plant, including the leaves, stems and leaf sheaths. The pustules rupture and a powdery mass of reddish brown spores are exposed. There might be more than 350,000 spores in a single pustule (over time). Stem rust pustules are larger than leaf rust pustules. Eventually, the reddish brown pustules turn black.
Verticillium and Fusarium Wilts
The barberry also suffers from verticillium wilt or fusarium wilt. The wilt is spread through the soil, infected seed and infected plant debris. When trimming barberry, be sure to remove all cuttings. Keep fallen leaves and fruit raked up. Discard all plant debris away from the barberry bushes and other plants.
Causes of wilts include overfertilization and weeds. Weeds infected with wilts transfer the disease to healthy barberry bushes. Choose a disease resistant variety (such as the Japanese barberry) when planning your barberry bush landscaping. Do not overwater and over fertilize the plants.
Leaf spot is another fungus that is attracted the barberry bush. Leaf spot shows up as black or brown spots and/or patches on leaves. The spots have yellow ringing the edges. Leaf spot spreads easily---through insects, people, dirty garden tools and even rain. To control leaf spot, wait until the plant is dry, then remove the infected leaves. Be sure to clean up any fallen or cut debris around the plant. Water barberry bushes from underneath to help prevent leaf spot. If the disease cannot be controlled, spray the barberry bushes with a fungicide recommended by your local nursery.