Raspberry spur blight is caused by the fungus ''Didymella applanata''. This
plant pathogen
Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomyc ...
is more problematic on
red raspberries ''(
Rubus idaeus
''Rubus idaeus'' (raspberry, also called red raspberry or occasionally European red raspberry to distinguish it from other raspberry species) is a red-fruited species of ''Rubus'' native to Europe and northern Asia and commonly cultivated in othe ...
)'' than on black or purple raspberries. The fungus infects the leaves first and then spreads to the cane. It causes necrotic spots on the cane near the base of the
petiole attachment.
Raspberry spur blight can cause a significant reduction in yield, fruit blight, premature leaf drop, and weak bud and cane growth. The magnitude of damage is not clearly understood in the United States, however, studies from Scotland suggest damage to the cane itself is limited.
[Ellis, Michael A]
"Spur Blight of Red Raspberries"
2008. The disease has minor economic impacts by reducing leaves in the summer or killing buds. Major economic damage occurs if the disease manages to kill the entire cane.
In the United States, this disease is found in Oregon and Washington.
Signs
In the late summer, the bark of infected areas split and the lesions produce fruiting bodies called
pycnidia
A pycnidium (plural pycnidia) is an asexual fruiting body produced by mitosporic fungi, for instance in the order Sphaeropsidales ( Deuteromycota, Coelomycetes) or order Pleosporales (Ascomycota, Dothideomycetes). It is often spherical or inve ...
. Pycnidia appear as small black dots to the naked eye and can be seen as flask-shaped structures under a microscope.
Perithecia
An ascocarp, or ascoma (), is the fruiting body ( sporocarp) of an ascomycete phylum fungus. It consists of very tightly interwoven hyphae and millions of embedded asci, each of which typically contains four to eight ascospores. Ascocarps are mos ...
, another fruiting body, forms after the pycnidia. Perithecia appear as medium, black, erupting dots.
In the spring, spur blight can be mistaken for winter injury therefore it is important to scout for these signs in September to assess the potential damage that will be done in the following season.
Symptoms
Infection generally occurs in the late spring when the environment is wet and optimal for fungus to proliferate. Symptoms do not become visible until mid to late summer. Visible symptoms include purple and brown expansive lesions appearing below buds, leaves and the lower portion of the stem.
leaflets that are infected have wedge-shaped brown patches. Infected leaflets may fall, leaving only the petiole and canes behind. During the next season, branch growth from diseased canes will often be weak and wilted.
[Heidenreich, Cathy. http://www.fruit.cornell.edu/berry/ipm/ipmpdfs/Raspberry%20cane%20disease%20mgmt.pdf "Managing Raspberry Cane Diseases", March 13, 2006.]
Disease cycle
The raspberry spur blight fungus spreads through the
pycniospore Pycniospores are a type of spore in fungi. They are produced in special cup-like structures called pycnia or pynidia. Almost all fungi reproduce asexually with the production of spores. Spores may be colorless, green, yellow, orange, red, brown or ...
s that are released from the pycnidia. The spores are released and infect other raspberry plants with the help of rain through open wounds or natural openings. The fungus will then spread throughout the plant and will live in lesions during the winter to survive. After winter there are two spore types that are formed pycniospores and
ascospore
An ascus (; ) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or s ...
s. The pycniospores come from
conidia
A conidium ( ; ), sometimes termed an asexual chlamydospore or chlamydoconidium (), is an asexual, non-motile spore of a fungus. The word ''conidium'' comes from the Ancient Greek word for dust, ('). They are also called mitospores due to the ...
and the ascospores come from the perithecia that was formed. Both of these spores will spread through rainfall onto new plants and start the process over again.
Environment
The environments that are favorable for raspberry spur blight are conditions that usually favor spore production. Infection generally occurs in late spring when the environment is wet and moist. Thick and dense patches of raspberries also increase infection.
Disease control
Methods of controlling this disease include improving air circulation to allow fast drying of the plant, removing weeds that slow airflow, reducing the number of wet periods the plant is exposed to, avoiding the use of fertilizers because it promotes growth of susceptible tissue targeted by the pathogen, thinning plants for better light exposure and removing possible sources of inoculum. Removing sources of inoculum can be completed by destroying wild brambles that may carry the disease, removing and destroying all previously infected material and using special fungicide in certain situations.
Other methods include starting with disease free plants, buying cultivars that are less susceptible to raspberry spur blight such as Brandywine, Killarney, Latham and Newburgh, and avoiding cultivars that have greater susceptibility such as Royalty, Titan, Canby, Skeena, Willamette, Reveille, and Sentry. Willamette readily infects with spur blight but is tolerant to the disease and still produces satisfactory yields.
It is important to minimize plant wounding and maintain proper soil nutrition. If you plan on pruning, always allow four to five days for healing before exposing the plants to water. Chemicals can be used in some cases. However, the spraying of chemicals is only efficient if it is sprayed on the entire raspberry plant. These include
lime sulfur
In horticulture, lime sulfur (British spelling lime sulphur) is mainly a mixture of calcium polysulfides and thiosulfate (plus other reaction by-products as sulfite and sulfate) formed by reacting calcium hydroxide with elemental sulfur, used in p ...
or
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
application,
captan
Captan is a general use pesticide (GUP) that belongs to the phthalimide class of fungicides. It is a white solid, although commercial samples appear yellow or brownish.
Applications
Although it can be applied on its own, Captan is often add ...
/
fenhexamid mixture (
Captevate 68WDG) applied when 8-10" shoot growth, and
Strobilurin Strobilurins are a group of natural products and their synthetic analogs. A number of strobilurins are used in agriculture as fungicides. They are part of the larger group of QIs (Quinone outside Inhibitors), which act to inhibit the respiratory c ...
s.
Recent research indicates that
chitinase
Chitinases (EC 3.2.1.14, chitodextrinase, 1,4-β-poly-N-acetylglucosaminidase, poly-β-glucosaminidase, β-1,4-poly-N-acetyl glucosamidinase, poly ,4-(N-acetyl-β-D-glucosaminide)glycanohydrolase, (1→4)-2-acetamido-2-deoxy-β-D-glucan glycano ...
s may effectively control against raspberry spur blight. Specifically, chitinases, when applied, were found to reduce lesion size, and control infection of internal tissues.
Notably, the application of
Phytoverm, a ''
Streptomyces avermitilis
''Streptomyces avermitilis'' is a species of bacteria in the genus ''Streptomyces''. This bacterium was discovered by Satoshi Ōmura in Shizuoka Prefecture, Japan.
The first complete genome sequence of ''S. avermitilis'' was completed in 2003. ...
'' metabolite, had a similar inhibitory effect on disease growth. Other plant pathogens have also been found to reduce the growth rate of casual agent ''D. applanata'' such as
entornopathogenic fungi (
Hyphomycetes
Hyphomycetes are a form classification of fungi, part of what has often been referred to as fungi imperfecti, Deuteromycota, or anamorphic fungi. Hyphomycetes lack closed fruit bodies, and are often referred to as moulds (or molds). Most hyph ...
).
Importance
The United States is the third largest country in producing raspberries. Raspberries are grown all over the country, but the majority are produced in Washington, California and Oregon.
However, in the United States the magnitude of damage caused by raspberry spur blight is not clearly understood.
References
{{Taxonbar, from=Q5274261, from2=Q7295200
Fungal plant pathogens and diseases
Raspberry diseases
Pleosporales