When you think of management, do you think of how to control diseases in the vegetable garden? Things like spraying fungicide, pulling out virus infected plants, mulching tomatoes, pulling weeds and cleaning up the garden at the end of the growing season? Well here is some good news. You can control rhubarb diseases simply by baking a pie or crisp. Harvesting rhubarb can actually reduce certain diseases. The trick is identifying the diseases and removing the stems that have them.
Rhubarb is attacked by several different leaf spot diseases. Two that you will see most often are caused by fungi. Ascochyta rhei leaf spots start out as light green or yellow. Those eventually turn into white spots with a red margin. Oftentimes the dead tissue at the center of the spot falls out making the leaves look like your kid has been at them with his BB gun. The other nasty is called Ramularia rhei. The rhei is an indication that it is rhubarb that it infects. The leaf spots from this stinker start as small red spots that eventually develop a white or tan center with a purple margin. This one can also cause infections on the stems. Those infections start as small spots then grow into sunken white or tan ovals.
Spores from both of these fungi are spread from existing spots to new leaves by wind, and we have had plenty of that, or splashing water. Removing the diseased stems from the garden will reduce the amount of fungal spores available to start a new infection. Selective harvesting can significantly reduce the amount of disease in your rhubarb patch.
Now you are ready to select stems for your culinary masterpiece. Pull stems that have leaf spots. Since the fungus can survive and produce spores on dead leaves, you will need to bag, bury or burn the leaves, never leaving them in the garden. Compost may not get hot enough to kill the spores so that isn’t a good option.
Leaf spot fungi are tough. They can survive the winter on old leaves so be sure to remove them from the garden in the fall. Rhubarb should be planted in full sun. It, like asparagus, is heavy feeder and should be fertilized as growth starts in the spring and again after the last harvest about early July. If your patch was touched by frost, discard any stems that have been frosted. They contain oxalic crystals. The leaves are full of this chemical and are poisonous. Not a problem for people and most animals won’t touch them.
Now is the time to go strolling though your iris. Put markers next to the clumps that have developed a dead middle or have invaded their neighbors. A large clump of blooming iris is a beautiful thing but not if it’s growing in the middle of your lily. While it is very tempting to get the shovel and remove the offending plant, and, unless you plan to discard it, wait until the end of August to transplant either iris or peonies. This gives them a chance to recover from blooming and get ready for next year. Now that your pie is done, sit out on the porch with a slice and enjoy the blooming iris.
Bev Johnson is a Master Gardener with the University of Minnesota Extension. Her column appears in the Weekend Edition.