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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 23, Number 3
July 1969

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Rhododendron Diseases
J. H. Tinga, University of Georgia, Athens, Ga.

        While I am not a disciple of Rachel Carson and her philosophy in Silent Spring I do believe she has something to say to us about the balance of nature. Therefore in these few lines I should like to speak to the problems of diseases and Rhododendrons. I should like to take a very broad perspective of diseases and in fact consider many things besides fungus root rots. In fact, as a horticulturist I am looking at the problem from the point of view of the plant rather than the point of view of Pythium or Phytophthora root rot fungus.
        You will marvel that I even spell the name of the topic different than the Plant Pathologist - I spell it dis-ease or not at ease. What can cause the Rhododendron plant not to be at ease in its habitation in your garden? I shall start with a list of non-pathogenic diseases.

(1) Selection of the right variety. Don't get too far out of the mild middle-Atlantic states if you do not have Iron Clad rhododendrons and don't get too far out of the south if you insist on Pericat azaleas. This is a very common dis-ease and you do not have to experiment with varieties very long until this dis-ease costs you money.
(2) Next is the particular site. This is probably micro-climate while (1) was a macro-climate situation. The north facing slope of a hill or the north side of the house, may be a very satisfactory place or site for a rhody or azalea. The south slope or side of the house may produce failure less than 50 feet away. The problem here is the rate of drying of the leaves in summer and especially in winter when the soil or stem may actually have non-flowing ice instead of liquid water in it.
(3) The water-air relations in the rhizosphere. That is a long name for root zone. Very commonly, plants are set too deep. Some of my gardener friends get all carried away by rhododendrons and azaleas in the garden centers at this time of year and they buy some more plants. And they enjoy them in bloom this year. But next July 1-14 is annual vacation time at the lake. The azaleas are not so pretty then, so forget the azaleas and on to the power boat and water skis. Unfortunately the investment in azaleas that the garden center salesman assured you would increase in value over the next 20 years went bankrupt. The azaleas died because they had a large top and not enough root system to pump in enough water from the dry soil. The next time you are in the mountains of North Carolina where the rhododendron becomes a dominant species, get out of your car and go up to a heavy natural planting and look on the ground and inside the great clump of foliage. You will see dead plants. They died from the severe drought of last year and the year before.
        I am trying to say that under natural conditions, on Roan Mountain, or in your garden, plants die from stress of all kinds. I remember a planting of 5000 azaleas at Dulles Airport planted near the terminal in March. They looked O.K. in April and May and June, but the rain of the dog star began in July. And it rained and it rained on that flat bed of azaleas. About 4500 plants died. You may say from root rot, but I say that the soil oxygen was replaced by water and the roots suffocated, or drowned as they say in Fairfax.
        Now I am depending heavily on my colleagues to talk about Phytophthora root rot. My approach is to make the environment favorable to Rhododendron and less favorable to Phytophthora and you can out grow the disease. The water-air relations are important here. If the soil is too wet for the rhododendron and just right for the Phytophthora, then you are allowing the enemy to win and you can apply Dexon all year, but unless you correct the basic trouble, you will lose the costly battle in your garden.
(4) Disease control by sanitation. This is the most difficult and at the same time the easiest method to control your problems. This also means a level of intensity of culture that is more artificial than natural. You start with clean soil and you keep it clean. You do not bring in a new variety from a friend or even from a rhododendron expert until it has been in quarantine to see if it has root rot or any other trouble. Some people get their plant growing area all disease-free and then reintroduce disease organisms which are secretly attached to the roots of new plants that are brought in. It is like unwittingly smuggling dope to your teenage daughter when you gave her a new brand of filter cigarettes. Not Virginia Slims but Mary Wanna. Be careful of new plants - of new mulch - of new soil and even of the dirty shoes of visitors from the American Rhododendron Society. They don't mean to contaminate your garden with root rot but they leave harmful fungus spores that they picked up in the compost pile where dead plants are thrown.
(5) Root rot control by soil acidity. Mr. Jim Wells of New Jersey has a very interesting series of articles on Rhododendron culture in the American Nurseryman. He says keeping the pH at 4.5 will result in unfavorable growth of Phytophthora and good growth of Rhododendron. Maybe gypsum is a better calcium source than lime for this dis-ease reason as well as soil reaction.
(6) A very important dis-ease is bugs. I include spider mites and lace bugs and stem girdlers and leaf miners. Consult your local insecticide source for control equipment and control poison concentrate and instructions on how to kill the bugs and not you. I still see gardeners using toxic concentrates without plastic gloves. Or if they have gloves they contaminate the inside of the glove with poison. Please use care with all toxic substances. This is the place where I am really chickenhearted. I got good and sick once, and one time is enough.
(7) A new prominent dis-ease is improper use of weed killer. 2-4D will curl azalea foliage in spring growth along with dandelions. Weed killers are good when handled with care.
(8) My final point is that we are quite unreasonable in our demands of an unfavorable and changeable natural environment to produce good flowers and leaves on rhodondrons and azaleas. If we want to make such high demands then we may go to an un-natural environment. Plastic greenhouses can extend our number of days of gardening pleasure. We can control petal blight by keeping the foliage dry: we can keep temperature and soil water at a more exact measure. So I commend to you to make a major change in your idea of what a garden is. If it is a high intensity area of un-natural varieties and un-natural soil and un-natural water and un-natural sanitation; why make demands on a natural situation. Increase plant growth and decrease disease with an unnatural environment. It will cost you twice as much but you will get 3 times the pleasure and benefit.


Volume 23, Number 3
July 1969

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