Black Spot (Diplocarpon rosae)

Plants/trees Affected

Roses (both cultivated and wild)

Symptoms

The appearance on leaves of round to irregular dark brown or black spots with fringed edges, followed by yellowing of the leaves and defoliation.  The spots can also appear on the thorns and stems.  If the rose continues to loose its leaves it will become weakened and susceptible to other stresses.

The fungi live on fallen leaves and sometimes soil through the winter.  In spring spores will be discharged and carried by the wind or rain to the new leaves.  The disease can also be transferred on hands, clothes and tools. The spores combined with water on the leaf allows the fungus to germinate and penetrate the leaf causing infection, often in just one day.

Black Spot thrives best in warm wet periods.  As the fungus develops the spots will appear and it may also lead to other types of fungi and secondary infections developing.  Black Spot is more common these days as towns and cities have cleaner air with less pollution and hence sulphur which used to inhibit fungal diseases.

Prevention

Try to purchase roses that are disease resistant and plant them in sunny positions with good air circulation and avoid overhead watering or water in early morning to allow time to dry.  Preventative fungicides can be used as soon as the leaves start developing and repeated during the summer months.

To prevent further infection remove and destroy all diseased leaves and tidy up fallen leaves at the end of the season.  Apply an organic mulch in early spring both to feed the plant and smother any spores that are over wintering on the soil at the base of the plant.

Treatment

• Remove all infected leaves and prune back hard.

• Treat the infected areas with powdered sulphur or

• Apply a chemical fungicide which includes one of the following :

Penconazole
Bifenthrin
Myclobutanil

Also use a protectant such as Mancozeb to prevent the fungus developing resistance to the fungicides.
 

Note – refer to manufacturers’ instructions before using any of the above chemicals.