Michelle Grabowski, UMN Extension Educator
Leaf spot fungi and bacteria come into the garden on infected seed or transplants or are blown in on the wind. Many of these pathogens can survive from one season to the next on infected plant debris. Splashing rain carries fungal spores and bacteria from the soil and plant debris onto this year's leaves. Moisture in the plant canopy then allows these pathogens to start new infections. Established leaf spots create a whole new generation of bacteria and fungal spores, starting the cycle all over again.
Luckily several basic cultural control practices can help to keep leaf spot pathogens in check.
1. Reduce moisture on leaves and fruit by watering the base of the plant with drip irrigation, a soaker hose, or simply by directly the hose at the soil and not the leaves.
2. Stake plants like tomatoes, runner beans, and cucumbers.
3. Mulch the soil with straw, wood chips or a plastic mulch to prevent the pathogen from being splashed up onto the lower leaves.
4. Inspect plants regularly. If a few leaf spots show up, pinch off the infected leaves and remove them from the garden. Never remove more than a third of the plants foliage!
5. At the end of the growing season, remove infected plants or till under the plant debris to speed up breakdown of infected plant parts.
6. Rotate crops. Wait 3-4 years to plant the same plant in the same location. It is best to rotate between plant families. Follow tomatoes with broccoli, corn or beans since they are not closely related. Peppers and eggplant should not follow tomatoes since they share many of the same diseases.
The leaf spot diseases below have been recently found in Minnesota.
Black Spot and Gray Spot - These two fungal diseases of brassicas are caused by Alternaria brassicae and Alternaria brassicicola. Disease can occur on cabbage, cauliflower, Chinese cabbage, broccoli, Brussels sprouts, kohlrabi, kale, turnip, and rutabaga. Leaf spots start out as small dark pinpoint spots, but quickly grow into a large gray to brown circle. Dark rings within the spot make them look like a target. Leaf tissue around the spots turns yellow, and dark brown spots may be seen on the heads of cauliflower andbroccoli.
The black spot and gray spot fungi can be blown into the garden on wind or brought in on infected seed. The disease thrives when high humidity occurs. To reduce problems with these fungal leaf spot diseases, remove diseased leaves from the garden and till in infected plants at the end of the season. Be sure to remove weeds from the brassica family because they can harbor these fungi even when vegetable plants are not around.
Early Blight - This common fungal disease of tomatoes is caused by Alternaria solani. The fungus can also infect potatoes and occasionally eggplant and peppers, but the most severe damage in gardens often occurs on the tomato plants. Fruit, stems and leaves can all be infected by the early blight fungus. Infection on all three plant parts results in large brown spots, with concentric dark rings, that look like a bulls eye.
Bacterial Brown Spot - This bacterial disease of beans is caused by Pseudomonas syringae pv syringae and can affect both the leaves and pods of the bean plant. Leaves have round brown papery spots. Leaf spots occasionally fall out, resulting in a shot hole appearance of the leaves. Infected bean pods also have brown spots and may be bent or twisted around the infected area. To reduce problems with bacterial spots on beans, space plants to allow good air movement between plants. Stake runner beans. Avoid working in plants when leaves are wet. Instead, wait until a cool dry day to pinch off infected leavesand pods. Remove this diseased material from the garden.
Septoria Leaf Spot - This fungal leaf spot disease of lettuce is caused by Septoria lactucae. Spores from this fungus often come into the garden on seed, but can also survive in plant residue and on some weeds. Gardeners that have problems with Septoria leaf spot of lettuce might consider looking for lettuce seed that is produced in a desert area, like the south western states, as these seeds are less likely to be contaminated by this moisture loving fungus. When growing successive crops of lettuce, be sure to seed the next crop in a location away from any currently diseased plants. Do not plant lettuce in the same area of the garden for one year to allow infected plant debris to break down.