Cedar-apple rust is an interesting and complex disease involving alternating hosts to complete its lifecycle. This disease is caused by the fungus (Gymnosporangium juniperi-virginianae). Now that is a mouthful to pronounce and something great to discuss with guests while at a party that you find boring and suddenly wish to impress someone. Maybe not, as perhaps they will think you have had too much wine or are some sort of scientific egghead. Better to just pour yourself another glass of wine! Unless you really are attending a symposium for plant pathologists.
Cedar-apple rust is of major concern to anyone who is trying to grow apples. It also affects large fruiting crabapple varieties being grown for fruit, ornamental crabapples and hawthorns too. All these hosts are related as all are in the rosaceous family of plants.
The secondary host plant for this fungal organism to complete its life-cycle is Eastern Redcedar (Juniperus virginiana) and its numerous cultivars (Canaert, Burki, Hill Dundee ect). While not so devastating to junipers (primarily cosmetic) this fungus is the bane of apple growers as their fruit is deemed worthless as nobody wants to eat infected apples that look bad.
When the fungus infects junipers, a small gall will form on the site of infection. The gall is usually ½”-1” in size and brown in color. The following Spring (usually in May in the upper mid-west), this gall will erupt with an orange gelatinous growth that has telial horns. This growth is so bright orange and weird looking it will always be remembered once you see one (looks like something out of a cheap science fiction horror film). Spores will eventually be released from these horns and can travel extremely far on wind currents. They can infect susceptible hosts (crabapples, apples, hawthorns) when conditions are right.
When apples, crabapples or hawthorn are infected, the infection site on the leaf turns into a yellow/orange spot. As the season progresses, this spot will develop further and forms tube-like structures. The fruit of apples and hawthorns can also be infected. Spores are eventually released and will germinate on eastern red-cedars if conditions are correct. This alternate host cycle keeps on repeating.
It should be noted that some apple varieties are more susceptible to cedar-apple rust than others. Also, not all junipers are host to cedar-apple rust. Chinese junipers (Juniperus chinensis) are not as susceptible to cedar-apple rust as are Eastern Redcedar and its numerous cultivars. Control of cedar-apple rust can be obtained by pruning off all galls found on junipers should they be infected. However, Eastern Redcedar is a native plant that is quite commonly found in abundance in the wild so it will be impossible to eradicate all sources of spores of cedar-apple rust.
While removal of Eastern Redcedars near orchard areas will help, control of cedar–apple rust will be best obtained by growing apple varieties that are less susceptible or by spraying with a preventative fungicide spray. I must stress that fungicide use will only be preventative not curative. To work, it must be applied before infection takes place on the apple leaf or fruit. The fungicide must coat the leaf/fruit to prevent the rust spore from germinating once it lands on the susceptible host plant. Normally, making two applications each spring should do the job. One should apply when the trees have leafed out but before full bloom and then again at petal fall or shortly thereafter.
The easiest and perhaps the best fungicide to use for home apple growers would be any fungicide product containing the active ingredient: myclobutanil. This is commonly sold as Immunox under one manufacturer’s brand and is available online or at some local retail stores. It must be noted that there are other fungicides that work too but be advised that not all fungicides will work at preventing cedar-apple rust. Also, not all fungicides that work on preventing cedar-apple rust are registered for use on apple trees. A good example is any fungicide containing the active ingredient chlorothalonil. It will work to prevent rust but is not registered for use on apple trees. It can be used on ornamental crabapples and hawthorns however.
There is a lot of misleading information on the internet about this. I must advise you that the pesticide label is the “law”. What is printed on it is there to guide you to get good results and assuming it is applied in a safe manner of application. The directions on the backside of the pesticide container will tell you what rate to apply, when to apply, what crop it is safe to apply to, and what disease issue can be prevented for each crop by using that fungicide. You will also find a statement that reads “It is in violation of Federal Law to use this product in a manner inconsistent with its labeling”.
It is extremely important to read ALL the information on the label before using. I say this because otherwise you may come to false conclusions. I will not list specific brands for example, but one brand of a certain fungicide packaged for the general public states “controls rust” on the front label. It also states “for fruit trees” on the front of the label. It even shows pictures of fruits on the front label. While both statements are true, this is only a brief summary of possible uses for this particular fungicide. The very detailed directions on the back of the fungicide bottle explain that it is legal to apply for cedar-apple rust control on ornamental crabapples only. Under the “Fruits” section of these same directions, one will also read that this product is only listed for use on plums, peaches and cherries (Not Apples). IT IS ILLEGAL TO USE THIS PRODUCT ON APPLE TREES!
Sorry for such detailed information on safety and legality of use. However, it is so very important to carefully read all the long list of information on the back pesticide label of any pesticide product before using so you choose the right product for using safely on apple trees or any other crop that you need pest control for. Especially more so for any food crop. My intention is not to scare you into not using a chemical control for cedar-apple rust on apple trees but to use a chemical control safely and wisely.
Should you wish to avoid spraying chemicals, some apple varieties are reported to be resistant to rust. It seems their degree of resistance will vary according to how heavy the rust pressure is in your growing area. Some of the varieties considered to be highly resistant are: Liberty, Zestar, McIntosh, Empire, Red Delicious, Macoun, Spartan, and Albemarle Pippen. You may want to consider giving these varieties a try if you are in an area where wild Eastern Redcedars abound.