Cassandra Swett, Assistant Professor and Extension Specialist – Berry Pathology, University of Maryland and Penn State University; clswett@umd.edu
We all get a little rusty as we get older, but one thing that we don’t want to see getting rusty is our brambles. There are several rust diseases that affect brambles. I’m just going to focus on orange rust, which is the most important rust disease in the Northeast.
We are definitely seeing a bit of orange rust this year, with the cool wet springs. You’ll see this disease on blackberries, black raspberries and purple raspberries. Orange rust does not affect red raspberries.
Orange rust is an unusual fungus—it grows systemically throughout the whole plant. So once a plant is infected, it will stay infected the rest of its life, and be a persistent source of inoculum for other plants. Over time, orange rust stunts and weakens plants so they will not bear fruit, but plants do not typically die. All in all, not a disease you want in your bramble field.
Orange rust stunting black raspberry plant. Note the “spindly” elongated shoots.
Orange rust is caused by two species: (1) Arthuriomyces peckianus, which is more common in the northeastern US. (2) Gymnoconia nitens, which is more common in southern states and primarily effects blackberry.
WHEN TO CONTROL ORANGE RUST: LIFE CYLE
Infection by orange rust occurs when it’s persistently wet (for more than 12 hrs in a day) and between 43°F and 72°F. The fungus cannot infect if it’s hot for most of the day or if it’s very dry. Above 85°F the fungus cannot infect at all.
The life cycle of orange rust is much more complex than your typical fungal pathogen, so I’m just going to boil it down to the simple take homes:
First: In a new field, orange rust can come in on infected plants, or can spread from nearby brambles—either other fields or wild brambles.
Second: Rust overwinters on infected leaves on the soil surface and on old canes, so if rust gets established your field, it will likely persist at low levels through the life of the planting.
Third: There are two periods of infection that are important to control:
- about 3-4 weeks in the spring, around the time of shoot emergence, after the last frost, and
- about 3-6 weeks in the fall, from the time when primocane growth slows until first frost
Understand that these are estimates—what really determines infection is the weather—again, persistently wet and between 43 and 72°F
Fourth: It is important to protect both leaves and emerging shoots/buds. The time of year and history of the field can inform you about whether you need to protect leaves, buds or both.
In the spring–protect against leaf infections if you are detecting rust for the first time; if you have a history of rust in your field, also protect against emerging shoot/bud infections.
In the fall–protect only against emerging shoot/bud infections.
This has to do with the type(s) of spore present in your field:
⇒ If you HAVE NOT had rust in your field in recent years, you should not have the overwintering spores, which infect buds. You should only have spring spores, which only infect leaves.
⇒ If you HAVE had rust in your field in recent years, then you probably DO HAVE overwintering spores, which infect buds.
CONTROL METHODS
- Scout and Remove Infected Plants in the Spring
Once a plant is infected, it must be removed. Otherwise it will continue to provide inoculum, allowing spread to other plants. It does not do much good to keep it, because after a couple of years the plant will stop yielding.
Spring is a critical time to scout for and remove orange rust-infected plants in the field, because this is the only time you will see the bright orange spores.
Scout early, as soon as new shoots start are emerging, after the last frost. Be particularly diligent when it’s a wet spring—this year is a great example. The disease is easily identified as orange pustules on the underside of young leaves. You will not be able to detect orange rust after sporulation ends (early to June, onwards).
Scout for orange pustules on the underside of young leaves, early in the spring.
- Chemical Control
Chemical control is an important compliment to removal. Once you remove all infected plants, you will want to spray to prevent the spores from infecting new plants.
When to spray
Weather can be a good indication that you need to spray—it has to be wet and between 43°F and 73°F to get infection. It is typically too cold between November and March and too hot between June and mid-August. In our region, the critical control periods for chemical protection are: (1) about 3-4 weeks in the spring, around the time of shoot emergence, after the last frost, and (2) about 3-6 weeks in the fall, from the time when primocane growth slows until first frost.
(1) Spring protection
Apply fungicides upon first discovery of the blisters, preferably before they burst open and release spores. If the field has a history of the disease, sprays should be initiated before blisters appear.
Direct this application to the foliage, since you are preventing leaf infections.
If you have had rust in previous years, ALSO do a spray directed to the base of the cane, to protect the developing buds from getting infected by overwintering spores.
(2) Fall protection
Apply fungicides if you detected rust in the field in the spring.
Direct towards the base of the cane, to protect the developing buds (both floricane and primocane); for floricane varieties—also spray the primocane shoots, to protect the buds on next year’s floricane.
What to spray
Rally (formerly called Nova) (myclobutanil)
Pristine 38 WG (pyraclostrobin + boscalid)
Cabrio 20EG (pyraclostrobin).
Fungicide recommendations for orange rust control, from the 2016 Midwest Fruit Pest Management Guide, developed by the Midwest Fruit Workers Group. The page numbers refer to the Management Guide—a link to the guide is provided below, if you would like to get more information.
Frequency of sprays
Apply on a 10 to 14-day schedule – use the shorter interval in wet weather. Alternate Rally with Pristine and Cabrio to prevent fungicide resistance. Do not apply more than two sprays without alternating.
An example of a 14 day-interval program for, say northern MD, would be:
In the spring–Starting after the last frost, at shoot emergence:
April 10: Rally 40WSP (Nova)
April 24: Cabrio 20EG
May 8: Rally 40WSP
In the fall—When primocane growth slows, until the first frost:
September 20: Rally 40WSP
October 4: Pristine 38WG
October 18: Rally 40WSP
November 1: Cabrio 20EG
Note that with high disease pressure, you would want to spray on a 10-day interval over these same time periods.
Some notes on these fungicides:
• Rally may have a bit better curative activity than the others because of its greater systemicity, which would make it the material of choice during or after a rainy period with inoculum already present. Do not apply more than 1.5 lbs (24 oz.) of Rally per year (label restriction)
• Since Pristine has two active ingredients, it has the broadest spectrum of activity.
• Avoid applying strobilurins (Cabrio or Pristine) more than three times each season, to prevent resistance development.
• While Abound (azoxystrobin) is labeled for use on brambles, it does not have orange rust (or any other rust for that matter) on the label.
- Resistant Cultivars
Red raspberries are all resistant. If you have persistent orange rust problems, this may be a good option.
Blackberry varieties reported to be resistant include: Choctaw, Commanche, Cherokee, Cheyenne
Susceptible blackberry cultivars include: Navaho, Ouachita, Chickasaw, Chester, Triple Crown. All black and purple raspberries are susceptible
Note: Triple Crown is reported as resistant in Kentucky trials, but it appears to be susceptible in our region.
- Site Selection
Avoid planting near woodlots or riparian corridors that have wild brambles.
- Clean Planting Material
Getting plant material from a clean source is critical to preventing establishment of orange rust in your field
RESOURCES
For additional information on orange rust and other bramble diseases:
Orange Rust on Brambles, by Mike Ellis. Ohioline.
http://ohioline.osu.edu/factsheet/plpath-fru-30
Midwest Fruit Pest Management Guide 2016. Produced by the Midwest Fruit Workers Group, out of Purdue University. Fungicide recommendations for bramble disease control.
https://ag.purdue.edu/hla/Hort/Documents/ID-465.pdf
Midwest Small Fruit Pest Management Handbook. Produced by Richard C. Funt, Michael A. Ellis, Celeste Welty, out of The Ohio State University. Comprehensive information on orange rust biology and cultural control.
http://pested.osu.edu/documents/CommStudy/2b%20Midwest%20Small%20Fruit%20Pest%20Mgmt..pdf
Mid-Atlantic Berry Guide. Produced out of Penn State.