Growing Healthy Plants
Damping-off, caused by many soil-borne pathogens, can cause
severe losses in new seedlings and transplants.
As many gardeners start planning for the spring and summer growing season, there are a few things to consider that can go a long way to ensure that you have healthy plants.
To Avoid Damping Off
Damping-off is a common seedling disease that can be easily avoided. There are several different organisms that cause the disease including Pythium and Rhizoctonia. Both of these survive on plant debris, in soil, and on wooden surfaces. When growing transplants from seed, it is essential to use clean trays and tools, to clean surfaces, and to use sterile soil. Pythium and Rhizoctonia can infect the roots of seedlings and the crown. The plants typically have dark brown to black roots and will fall over.
To minimize infection, clean growing surfaces, pots, trays, and planting tools with a 10% bleach solution and remove all dirt from the previous year. The use of a sterile planting mix
is best. Place pots and trays in individual saucers to isolate infections and prevent spread of the pathogens. The seedlings should be kept moist, but never excessively watered or left
in water. Damping-off is often worse where plants are kept very wet. The surface where pots and trays are placed also needs to be kept clean; preferably disinfected with a 10% bleach solution. If trays or pots are placed on the ground, put a sheet of plastic down to put the trays on. Both Pythium and Rhizoctonia are common soil pathogens and can come into contact with seedling roots as the roots grow into the ground, or water can carry the pathogens into the trays.
When purchasing vegetable or ornamental transplants, it is important to inspect the plants. Transplants should look healthy and vigorous. Stunted, yellow plants may lack nutrients, or have something more serious such as root rot or root infections with nematodes. By buying infected transplants and placing them in yards and fields, nematodes and root pathogens can be introduced that contribute to or cause problems in coming years.
For example, do not purchase transplants showing signs of powdery mildew or rust, or roses with black spot on leaves or stems. These plants have diseases that need to be treated
with fungicides throughout the growing season. Some powdery mildews and rusts infect other plant species and these pathogens can be introduced into yards and fields. The fungus that causes black spot on rose (Diplocarpon rosae) can survive on stems and buds during the winter and infect new leaves as well as neighboring rose plants in the spring.
Crop rotation is important to reduce disease incidence. When the same plant species is grown year after year in the same location, pathogens such as Fusarium can build up large populations in the soil. Instead of losing one plant as in previous years, all the plants can be lost.
Fusarium species are often very specific to one host plant. Species of other soil-borne pathogens like Pythium, Rhizoctonia, Verticillium or Phytophthora can often infect multiple plant
species. If it is known that one of these pathogens caused a problem in the previous year, determine its host range before planting. A Google search for the specific species of a pathogen and host range can provide this information or you can contact me (email@example.com).
Whenever possible, select varieties resistant to specific pathogens to avoid problems. Many seed companies provide information on their seed packets indicating resistance to certain pathogens. A tomato variety, for example, would have some of the following letters (from bonnieplants.com) to indicate resistance to the specific pathogens:
- V - verticillium wilt
- F - fusarium wilt ( two F’s indicate resistance to both races 1 and 2 )
- N - nematodes
- A - alternaria stem canker
- T - tobacco mosaic virus
- St - stemphylium (gray leaf spot )
- SWV - tomato spotted wilt virus
- LB - late blight
-Claudia Nischwitz, Plant Pathologist