Features Agronomy Diseases
Fusarium diseases common in Alberta

Field surveys over the last several years have confirmed the widespread occurrence
of fusarium species, which can cause seed-piece decay, wilt and dry rot in potatoes.

A project was started in 2004, with a follow-up in 2005, to see how common
fusarium diseases were in Alberta potato fields. This study was undertaken in
the wake of fusarium disease outbreaks in some parts of southern and central
Alberta in 2003. The project, headed by plant pathologist Ron Howard with Alberta
Agriculture, Food and Rural Development (AAFRD) at the Crop Diversification
Centre South at Brooks, included a storage survey for fusarium contamination
and dry rot of seed tubers, a field survey for wilt diseases and two fungicide
seed piece treatment trials in field plots.

"Based on the survey, we saw that fusarium diseases were widespread and
that growers need to use integrated management practices to help control these
diseases," says Howard.

In the seed survey, 20 tuber samples of seed potatoes were obtained from growers
or purchased in the marketplace. The samples represented the main varieties
used for commercial production and included different certification classes.
Background information on the seed production and disease history was obtained.

The samples were split in half and one half was analyzed for fusarium species
by washing the tubers and plating the suspension. The other half was assessed
for dry rot after four months of storage at 15 degrees C. All the seed lots
had dry rot and were contaminated with fusarium. However, there were wide differences
in infestation levels between lots. Dry rot incidence ranged from one to 100
percent of tubers infected.

Surface contamination of tubers with fusarium also varied widely, and there
was a direct correlation between the level of surface contamination and the
incidence of dry rot in stored tubers.

Fusarium sambucinum was the most common fusarium species isolated from
diseased tubers. At least three other fusarium species were present and included
F. solani and F. oxysporum.

"I think it is safe to say that F. solani and F. oxysporum
have an opportunity to cross over into other crops where they may cause root
rot and seedling decay," says Howard. "F. sambucinum is more
important for potatoes and less of a pathogen on other crops."

Crop rotation is especially important in helping to minimize these devastating
potato diseases. With crop rotations of at least four years for potatoes, the
diseases remain at lower levels and are not as specific to the potato crop.
"After you grow a crop on the same land for a number of years, the pathogen
population may become more specific and damaging to that crop," explains
Howard. "It is important to have good crop rotations when growing potatoes."

Wilt prevalent in southern
Alberta Seventeen processing potato fields in southern Alberta were also surveyed
in the summer of 2004. Howard says 16 of 17 fields had wilt symptoms. Verticillium
dahliae caused substantially more wilt in 16 of the fields, compared to
fusarium species, which were isolated in five fields in 2004.

A Colletotrichum species, likely C. coccodes, which causes black
dot disease, was isolated from 13 fields. With the cooler growing season in
2004, Howard says that verticillium wilt and black dot may have been favoured
over fusarium wilt, which seems to be more prevalent in warm, dry years.

Fungicides improve emergence and yield
Howard compared six registered and two experimental seed piece treatments on
Ranger Russet potato in field trials at Brooks and Edmonton. The seed pieces
were naturally infected with fusarium species, and a small amount of F. sambucinum
was also applied to the seed pieces in a second treatment. Compared to the inoculated
control, several fungicides improved emergence and total yield. However, some
of these differences were not statistically significant.

Dithane M-45, Genesis MZ and Maxim MZ generally showed the best control of
seedborne fusarium.

Howard says that in addition to treating potato seed pieces with a fungicide,
a wide range of other management practices also help to prevent or minimize
seed piece decay, wilt and dry rot. The disease may initiate from infected seed
or from pathogen inoculum present in the soil. Planting bruised or non-suberized
seed pieces may allow the pathogen to enter and cause seed rot. He says that
avoiding planting in extremes of dry, cold, or soggy soil will reduce infection
and improve potato emergence.

Avoiding wound to tubers at any stage of the cropping cycle helps minimize
wilt development in the field or dry rot in storage. Howard also says potato
production on lighter soils is best because heavier soils may restrict root
growth and favour disease infections. Similarly, land free of detrimental herbicide
residues, such as dicamba or picloram, will allow the potato plants to grow
more vigorously and tolerate or resist diseases.

Howard recommends proper harvest timing to help ensure the tuber has a tougher
skin that resists bruising. Top-killing may be required to ensure good skin
set. After harvest, wound healing can be promoted with proper storage; storage
temperature of 10 to 13 degrees C and relative humidity at 95 percent with good
air circulation for 10 to 14 days. Then reduce the temperature to recommended
levels for storing seed, table or processing potatoes. Fungicide treatments
as potatoes go into storage may also help reduce dry rot.

"There are many integrated practices that producers can use to head off
disease," says Howard. "Start off with good seed and follow all the
recommended practices because, based on our surveys and field trials, fusarium
dry rot, seed piece decay and wilt are common problems that may pose a threat
to seed, table and processing potato crops." -30-