The Black Spot seeks shelter in fallen leaves and diseased branches through the winter, waiting for warmer days to launch their attack. Raindrops splashing on soil under plants and brisk spring winds give these nasty fungal spores a lift, catapulting them directly onto unsullied, budding leaves. Besides being unpleasant to look at, they can damage leaves and inhibit blooms. Black spot spores spend the spring and summer beefing up their troops through reproduction and continuous movement to new plant parts.
IF CONDITIONS ARE RIGHT, BLACK SPOT SPORES CAN BECOME INFECTIOUS IN JUST ONE DAY, WITH NOTICEABLE EFFECTS IN LESS THAN A WEEK.
Here's how you can stop seeing spots. Start scouting for spots in spring when temperatures begin reaching 60F and the days are wet. Before black spot can infect plants, the temperature must be in the mid-60's and leaves must be wet for 6-9 continuous hours.
Carefully search leaves for spots, being sure to check densely clustered leaves since humidity will be higher where leaves are closer together. Also examine leaves close to the ground.
Take note of what you see and record changes in numbers or size of spots. Some spots may be left over from last year.
Black spot is difficult to get rid of after it has set in, even with chemicals.
Your best chance at warding it off is by making your plants an undesirable place to set up camp. Prune to decrease humidity and increase sunlight around susceptible plants.
Think about your aesthetic standards. Black spot is an ugly assaulter and will deface your plants mercilessly, but the plants will still most likely survive.
Continued leaf loss weakens plants and invites aphids-a one-two punch.
The health of the victim is key in determining if it will survive the assault. Plants that are weak or under stress may be killed by black spot.
You can also prune out invaded leaves and canes. Do not compost infected parts. For roses, prune victimized canes down to two buds. Give your plants an advantage by planting in well-drained, sunny places. Also, plant on the windy side of your house to let breezes dry out leaves and lower humidity near plants.
Don't buy wimpy plants! Purchase varieties that can fight off black spot.
Give plants a chance to air out. Don't plant them close together.
You can also destroy black spot hideouts by raking up leaf matter around plants frequently, especially in the fall. Don't compost these leaves.
Try also to smother the spores in the spring by putting mulch around plants.
And, avoid overhead watering. If that's not an option, do it in the morning so plants can dry out during the day.
Remember not to put plants from the same family in the same place every year since diseases usually assault plants from the same family. Rotate plantings from different families.
For fungicidal treatment, start in early spring to prevent black spot attacks. Be sure to treat vulnerable new growth. Usually treating once per week is advised, but read and follow the directions on the label. If your plants are already victims of black spot, you might try sulfur products to keep the attack under control. Use them when the temperature is between 65-85F. Do not use within 30 days of an oil spray. Always read and follow the directions for use, storage, and disposal when using chemicals.
Don't forget to evaluate the results
Keep an annual record that you can refer to each year. If certain plants have been seriously assaulted in the past, your notes will help you decide if preventative steps against black spot should be taken in the spring and which strategies have the highest success rates.