|
||||||||||
| Department of Agriculture, Aquaculture and Fisheries |
||||||||||
|
||||||||||
|
The fungus, Phytophthora infestans, causes late blight of potatoes which is a destructive disease in this region of Canada. Disease symptoms usually first appear on older (lowest) leaves soon after flowering, following wet or overcast weather conditions. Dark green, water soaked areas at the leaf margins spread inwards and later become dark brown and brittle. The edge of the lesion has a fluffy white growth visible on the underside of infected leaves when humidity is high or in the morning following a dew. This fluffy mildew produces sporangia which are spread by rain or wind to other plants. When conditions are wet or humid, disease spread can be extremely rapid, causing progressive defoliation, plant death, yield loss and tuber infections. Tubers near or on the soil surface often become infected with irregular sunken lesions, often appearing around the eyes. The infected tissue, granular and reddish in appearance, penetrates up to 2 cm into the tuber. Storage of diseased tubers can result in infection of other tubers causing further crop losses. The fungus can only survive between growing seasons (overwinter) as mycelium in living potato tissue. Infected tubers used for seed or discarded onto cull piles at farms and commercial storages or infected volunteer potatoes that overwinter in the field in provinces with milder climates or where snow cover occurs early in the fall are sources of infection for the new growing season. Airborne sporangia produced on infected plant parts are capable of travelling considerable distances (farm to farm and even province to province) during the growing season.
 Control Destroy cull piles by composting, burying or freezing and eliminate volunteers in the field with herbicides and proper crop rotation practices. Use seed potatoes that are free of late blight. The cultivars Brador, Kennebec, Nooksack and Sebago are resistant to some blight races. Foliar fungicides (see Publication No. 1300A) provide effective disease control but must be applied according to label instructions. Use of disease forecast announcements and crop inspection reports can reduce frequency of sprays during conditions unsuitable for the development of the disease. Tuber infections can be reduced by applying fungicides at and following top desiccation until all plants are dead. Many commercially available fungicides are contact or "preventative" pesticides (see publication #1300A). Some are systemic and "curative" but continued use of "curatives" can lead to resistance to the fungicide in the pathogen. |