Hi there, I am in zone 6b and have a clay soil. This year I had many diseases present in the garden, botrytis, blight, powdery mildew, maybe downy mildew etc. I am planning to plant tulips, daffodils, muscari, iris reticulata and crocuses. We may move in the next few years and if I move some of my bulbs, as well as my calla lilies, dahlias, bearded irises with meis there a way to treat them so that any soil diseases are not spread to the new garden? Also when is the optimal time to sow tulips, iris reticulata, crocus, and snow drop here?
You are quite right to be concerned about the possible transfer of plant diseases when moving plants from one location to another. So often plant diseases, pests and invasive species are moved into new locales along with transplants. The good thing about moving bulbs instead of whole rooted plants is that it is easier to disinfect the items prior to transplanting.
There are different species of botyris such as Botyris cinerea that is very common and infects a variety of plants like dahlias and begonias, and Botyris tulipae which is specific to tulips and lilies but can spread to members of the same family. Both Dahlias and Irises are prone to Botyris blight during wet, cloudy weather. There are biofungicides containing Bacillus subtilis that can help to control disease spread. The links below provide more information about Botyris Blight and Botyris tulipae.
Hot weather and cool nights promote another fungal disease, powdery mildew. Preventing stress from drought during hot weather is beneficial. You can also spray with an organic fungicide that contains Bacillus subtilis . On the other hand, Downy mildew, thrives during cool moist weather. It can affect a wide range of plants and is difficult to control. Copper based fungicides such as Bordeaux mix are sometimes used on woody plants. When using any fungicide follow the directions on the container carefully.
To avoid using any chemicals following good garden practices is really helpful in controlling the spread all of these diseases. These practices include:
- providing good air circulation, ie not overcrowding,
- watering well and mulching to prevent stress from drought (also avoid wetting the leaves),
- growing disease resistant cultivars,
- monitoring for insect pests that can carry diseases from one plant to another, and
- removing all diseased leaves and whole plants where necessary. (Do not compost any diseased plants. Instead place them in a plastic bag to dispose of with the trash.)
All of these practices are recommended procedures for controlling the spread of the diseases you mentioned – specifically No.2 drought prevention for powdery mildew and avoiding wet leaves for downy mildew. No. 5 is a must for preventing the spread of any diseases. Along with removal of diseased plants, also remove any garden debris around the affected plants that can harbour the diseases.
When you are planning to move your bulbs, the best time to dig them up is after they have flowered and the leaves have turned brown and are dried. This allows the bulbs time to replenish their food stores so they can grow and bloom the following year. It will take about 6 weeks after blooming. Deadhead the plants to prevent seeds from forming and using up more energy from the bulbs. Remove all the dead leaves and stems. Dig them up carefully to avoid damaging the bulbs and let them dry for a day or two, preferably on an wire screen for good air circulation. Brush off any remaining soil and apply a fungicide. Store in labelled netted bags or in a cardboard box in layers separated by crumpled paper. I expect that you already have experience in digging and storing your dahlia tubers. When you are ready to move make sure the tubers are well hosed off, dried and treated with fungicide as well before storage. I am including a link to a good article about fungal control in dahlias which describes in detail the various diseases and comments on fungicides. Note that the author writes that fungicides are not the final answer for disease control and that garden sanitation is the most important factor in controlling the spread of diseases.
Although bulbs like crocus, tulips and snow drops can be planted quite late in the fall, even into November, now is a good time to plant them. Narcissus bulbs prefer to be planted earlier in the fall so plant them now as well. When planting bulbs in a heavy clay soil, keep in mind that clay soil holds more water during wet weather and bulbs sitting in soggy soil will rot. Adding compost or leaf mold will improve the soil by allowing more air space between the soil particles. Here are links to two sites with more detailed information about planting bulbs:
As for the iris reticulatas they can also be planted now. The site below has good information about growing iris reticulatas.
If you are interested in learning more about plant diseases and pests and how to control them organically, a very good source book is:
The Organic Gardener’s Handbook of Natural Pest and Disease Control edited by Fern Bradley, Barbara Ellis and Deborah Martin and published by Rodale Press.