Pesky Blackberry Foes

Here in North Carolina, blackberry season is in full swing! For many North Carolinians, that means it is the perfect time to stock up their freezer or pull out those ball jars and can up some tangy black berry jam. Blackberry season brings back sweet summertime memories of riding my bike down the road to the black berry thicket at the edge of the woods, gorging myself with fresh, juicy berries (probably picking up a few chiggers in the process), and heading home with purple stains on my hands, face, and clothes. Whether you prefer to pick your own berries or buy a pack from the local Farmer’s Market, we can all agree that blackberries are a signature snack for a late summer’s day. Unfortunately, blackberries, like all things delicious, come with their fair share of pests and diseases that impact fruit production.

This summer, we have seen two similar but different diseases on blackberry samples: orange rust of blackberry and black raspberry and leaf and cane rust of blackberry. Orange rust is typically the more devastating disease because it can become systemic, moving from leaves into other parts of the plant. The orange rust fungus has two forms, Arthuriomyces peckianus (formerly Gymnoconia peckiana) and Gymnoconia nitens, which differ only in the number of spore stages produced. Pustules full of orange-yellow spores develop on the undersides of leaves in late May and early June. These spores are blown to healthy leaves and infect when humidity is high and leaves are wet. Heavily infected leaves may die and defoliate. Once the plant is infected, the rust fungus becomes systemic. It grows down the infected shoot, into the crown, and then can enter newly formed roots. Symptoms associated with shoot infections include proliferation of shoots, weak and spindly canes, and lack of spines on the shoots. In mid- to late summer, brownish black pustules that contain dark teliospores develop on the undersides of lower leaves. Teliospores do not infect, but germinate to produce basidiospores that can infect new buds or shoots, or the teliospores can overwinter on leaves before producing basidiospores the following year. Infected plants remain infected throughout their lifetime and do not recover.

Orange rust pustules on underside of leaves. Note leaf distortion. (Photo: PDIC Database)

Orange rust does not kill the plant outright, but infected plants are completely lost to production due to their inability to produce blossoms and berries. Controlling orange rust is largely achieved through cultural practices. Plant disease-free stock plants, eradicate diseased plants and wild berries in the surrounding area, and completely remove and destroy the entire plant as soon as symptoms develop on canes or leaves. Thin healthy plants to promote air circulation and to reduce leaf wetness.

Leaf and cane rust is caused by the fungus Kuehneola uredinis. Leaf and cane rust produces yellow spores in pustules that split the bark of infected canes, causing them to become weak. The pustules can also be found on the undersides of leaves. Diseased old canes should be pruned after fruiting. Alternate-year fruiting programs can help reduce disease pressure, and routine fungicide spray programs may be effective in preventing new infections. 

Leaf and cane rust. Note yellow spores bursting from cane. (Photo: NCSU Database)

Leaf and cane rust.  Infected leaves maintain shape unlike
those infected with orange rust. (Photo: NCSU Database)

Care must be taken to differentiate systemic orange rust from leaf and cane rust because leaf and cane rust does not require drastic removal methods to control disease. Identification of the rust pathogen requires a microscope and considerable knowledge in rust morphology. Suspect samples should be sent to the Plant Disease and Insect Clinic for a formal diagnosis.

Asiatic Soybean Rust Update

Reprinted from North Carolina Pest News
Steve Koenning, Extension Plant Pathologist
Jim Dunphy, Extension Soybean Specialist

Asiatic soybean rust has been confirmed in Robeson and Johnston counties, North Carolina. Between this find, and the confirmation of rust in Robeson County, North Carolina, and in Union County, Georgia, this puts rust approximately 105 miles from Charlotte, 140 miles from Elizabeth City, 15 miles from Fayetteville, 15 miles from Murphy, 35 miles from Raleigh, 80 miles from Washington, 75 miles from Wilmington, and 80 miles from Winston-Salem, North Carolina.

We do not recommend spraying soybeans with a fungicide to control Asiatic soybean rust if they are not yet blooming, if they are blooming but rust has not been confirmed within 100 miles, or if full sized seeds are present in the top of the plant (stage R6). Such pre-bloom applications have seldom improved yields, and repeated applications will likely be needed to provide season-long protection against rust. The higher labeled rates tend to provide more days of prevention, and may thus require fewer applications. The triazole fungicides, alone or in combination with a strobilurin fungicide, will probably provide better prevention of rust than a strobilurin alone. Be sure to check the fungicide label to see how many times it may be used in a season.

An exception to the above recommendation is if Asiatic soybean rust is found on the farm before bloom, spraying a fungicide to the rest of the fields on the farm is recommended. Soybeans that have just reached full bloom (stage R2) typically have 65 days until they’re safe from rust or frost (stage R7) if they are full-season soybeans, or closer to 55 days if they are double-crop soybeans. If they have small pods in the top of the plants (stage R3), they have 55 and 47 days, respectively, to R7. With full sized pods in the top of the plants (stage R4), they have 45 and 38 days, respectively, until R7. From stage R5 (small seeds in the top of the plant) they typically have 35 and 30 days, respectively. From stage R6 (full sized seeds in the top of the plants), they typically have 20 and 17 days, respectively.

Rust will typically take 10 to 20 days from initial infection to develop to detectable levels. It will take another 7 to 14 days to spread to other leaves on the same plant, and another 10 days to cause significant defoliation.” This assumes optimal conditions for rust, “65 to 85 degrees, and either overcast or rainfall” through much of this period. This is not common in North Carolina in September and October but has and can occur.

The rust prediction models say there was a fair to good chance rust spores were deposited in North Carolina this weekend. If so, we expect to detect rust in about three weeks in sentinel plots, which would be about October 1. It will likely take another two weeks with optimal conditions for rust to increase to damaging levels.

The current status of soybean rust in the U.S. can always be found at http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi.

Fusiform Rust: What's that orange stuff on my pine tree?

Just last week, I noticed the galls on the pine trees in my back yard started bursting open with the bright orange spores of fusiform rust. Fusiform rust is a damaging disease in landscapes and forests of the South. The most obvious symptom is spindle-shaped swellings or galls on the branches or trunks of pine trees.

Fusiform rust is caused by the fungus Cronartium quorum f. sp. fusiforme. Like many rust species, this fungus produces five different spore types and completes its life cycle on two different hosts: pine and oak.

During the spring, the fungus produces bright orange spores like the ones I observed on the galls. Typically, these spores are produced from late March to mid-April. Wind-blown spores infect newly formed leaves of several oak species, especially water, willow, and laurel oaks. Symptoms on oak are not conspicuous and usually the tree is not harmed. The fungus produces a different type of orange spore on the under-surface of oak leaves from late April through the middle of June. These spores are then wind-blown to nearby pines where they create new infections, causing new galls and continuing the cycle.

Pine trees are highly susceptible to Fusiform rust when young and infections that occur within the first 5 years of growth usually result in death. Later, main stem cankers can girdle and kill the tree or reduce its value. Stems with cankers are weak and susceptible to wind and ice breakage, and galls easily catch fire and stay afire. Secondary infections by the pitch canker fungus (Fusarium moniliforme var. subglutinans), black turpentine beetles (Dendrodoconus frontalis) and coneworms (Dioryctria spp.) aggravate the tree’s weakened condition, causing further damage.

Over the years, fusiform rust has become an increasing problem, particularly in pine plantations. Loblolly and slash pines are the most susceptible tree species. Longleaf is fairly resistant and shortleaf is highly resistant.

Management options include spraying fungicides, planting longleaf or shortleaf pines in areas with histories of severe rust infections, pruning branches with rust galls less than 15 inches from the stem, eliminating nearby oak hosts, and using rust-resistant clones.

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