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In other parts of the world, the dahlia is known as the Queen of Fall and offered only a short - but extraordinary - period of reign. Not so in Western Washington, where its rule is sustained from early summer well into November’s killer frost. Yet it is quite common among devoted dahlia growers that - at the end of show season - thoughts turn to digging and harvesting dahlia roots in October to avoid the unpleasant task of removing the slimy foliage during inclement and cold weather. Another consideration is the prevention of fungus or bacterial disease that often develops under those conditions.
Usually the first unwelcome guest, powdery mildew appears just when late August brings out the best blooms. Hot days and cooler nights encourage mildew growth on top of foliage. Later it may be joined by another fungus, entyloma, whose circular colonies infest lower leaves in particular. This dahlia smut needs your attention, since its spores winter over in the soil. Removing excess foliage and diseased leaves also opens the plant to better air circulation.
Dahlia growers also note that botrytis can affect young plants that are over-watered. Fusarium wilt may cause visible damage and partial weakening of foliage, but it also can lead to complete collapse of the dahlia. The bloom of botrytis is easily detected, so that the grower can apply remedies. Fusarium, on the other hand, exhibits different symptoms (such as discoloration, necrotic areas, or stunted growth). Warm temperatures and humidity, over-watering, and poor drainage may lead to wilt spreading.
The most pernicious fungus, though, came into my garden years ago. One dahlia was infected by sclerotinia, whose mycelium grows rapidly inside the hollow stem of dahlias. As it matures, the fungus causes collapse of a branch here or there, but it eventually will kill the entire plant. Its sclerotia look like black, hard pellets as they develop within the stem. Allowing these to fall to the ground ensures their survival. Like other mushrooms, they develop into fruiting bodies with a wealth of spores. If a dahlia branch exhibits signs of distress, look at the juncture of branch with stem. When you find a brown area there, suspect sclerotinia, remove the branch and investigate further. Since excessive moisture is often the cause of sclerotinia and those other fungi, reducing overhead watering is necessary. And in all those cases, infected foliage should never be composted but consigned to trash.
New dahlia growers mention the appearance of gall diseases, suspecting that these bacterial diseases may be spread by commercial mass marketing of dahlias with little understanding for plant health. The topic is still under investigation, but let me refer you to my recent article posted on the ADS website under the “Know” tab entitled You’ve Got Some Gall! The valuable research by Oregon State University’s Melody Putnam also can be found online; her articles offer numerous examples and photos of these and provide effective remedies.
The past four years have taught us all a lesson on transmission of infectious diseases, such as Covid viruses. No longer can we take lightly our parents’ admonishments to wash our hands, cough into elbows, and clean counters and door knobs. In the past I admittedly was rather casual in cleaning or sterilizing dividing tools. After all, several different approaches were proposed, and the formula for bleach/water solutions seemed to be based on personal preference, rather than any empirical evidence. Moreover, using bleach certainly corroded and damaged those tools, so I was hesitant to follow. But as our knowledge of dahlia pathology increases, we need to steer away from home remedies and follow the science. Like others, I now sterilize my pruners and other cutting instruments in Virkon-S, a veterinary disinfectant that also is claimed to be an effective agent against virus contagion.
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