Soil Fungus for Roses

Black Spot

• Black spot is surely among the most common fungal diseases that cause problems for rose growers. Caused by the fungus Diplocarpon rosae, the earliest symptoms of the disease are the titular black spots that appear on rose leaves. The black spots are ringed by a yellow halo, and infected leaves eventually withers and defoliates. Control black spot with a captan, chlorothalonil, mancozeb or thiophanatemethyl fungicide combined with effective management practices, such as pruning away diseased leaves, canes and other parts of the rose bush and destroying the parts right away.

Powdery Mildew

• Powdery mildew is caused by the fungal disease agent Sphaerotheca pannosa and is characterized by new rose leaves turning purple and curling inward. As the disease progress, a noticeable white, powdery substance covers the leaves and other parts of the rose; this is the telltale sign of powdery mildew. Again, pruning away diseased parts of the rose plant and destroying them helps control the spread of the disease, but serious infections should be treated with a lime-sulfur, sulfur or triforine fungicide.

Cotton (Texas) Root Rot

• A soilborne fungus called Phymatotrichopsis omnivora or Phymatotrichum omnivorum causes root rot, a serious disease that can be fatal to your rose plants. Infection begins on lower leaves and can spread to the taproot. Cotton root rot is a particularly difficult fungal disease to treat, and only a qualified professional should make a diagnosis. Symptoms resemble those of drought stress, and a diagnosis can only be confirmed by observation of microscopic fungal mycelium of rose roots. Typically, the rose grower simply has to replant new roses in a different part of the garden that is free of the pathogen.

Soilborne Fungi Concerns

• Fungal diseases are highly region specific, so some diseases may be uncommon in your geographic area and others may be more prevalent. The treatment of diseases cause by soilborne fungi is made difficult because fungi often persist in the soil for years. Therefore, if growers only treat the plant, chances are good that fungi living the soil will continually infect plants over successive growing seasons, often forcing the grower to plant new roses in different soil.