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Late Blight

Thursday, September 10th, 2009
Symptom of late blight on the potato leaf.
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(Phytophthora infestans)

Late blight is disease occurs while growing potatoes and caused by Fungi.
Although control measures exist, late blight remains the most serious fungal disease in most major potato production regions.

Symptoms of Late Blight

Water-soaked lesions appear on foliage that, within a few days, becomes necrotic, turning brown when dry or black when wet.

Under damp conditions, white mildew like sporulation is visible, especially on the lower surface of leaves. A pale yellow margin often forms around leaf lesions. Lesions on stems and petioles are black or brown.

Stem lesions are brittle and stems frequently break at the point of the lesion. Under certain conditions, wilting can occur on stems with lesions.
Disease is favoured by temperatures between 10 and 25°C, accompanied by heavy dew or rain.
POtatoes tubers infected by spores washed by rain from the leaves and stems into the soil have brownish surface discoloration.

Sections cut through tubers show brown, necrotic tissues not clearly differentiated from the healthy portions. Later secondary rot organisms develop in blighted tissue and rots spread in storage.

Management

Sources of inoculum are neighboring fields of potato or tomato, volunteer plants, and cull piles.

Riptide 6 Week Pest ControlThe last two sources can be removed. Soil survival occurs wherever the sexual stage (resting oospores) occurs as a result of the presence of both the A1 and A2 mating types, which can lead to early infections.

Once infection occurs in a field, control is a function of host resistance and spraying, mediated by the environment.


Potatoes growers should check with local extension agents for information on forecasting systems or spray schedules that maximize fungicide efficiency and resistance levels of cultivars. There are cultivars with various levels of resistance.
Both protective and systemic fungicides are available, but the latter should only be used according to regional or national strategies that have been developed to minimize the development of resistance in the pathogen.
To prevent tuber infection when growing potatoes, plants should be well hilled, foliage completely sprayed during the growth period, and vines permitted to mature and die naturally or be killed before harvest.

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Bacterial soft rot potato disease

Tuesday, August 4th, 2009

soft-rot-potatoWhile growing potato gardeners face problem of soft rot potato disease. Soft rot potato disease is caused by Erwinia carotovora or Erwinia chrysanthemi (Pectobacterium sp. and Dickeya sp). This potato disease spreads by seed tubers, insects, or irrigation water.
Other than sanitation, planting seed not infested with Erwinia and irrigating with clean water, there are few control options for soft rot.
Potato tubers with soft rot symptoms can dry out and the rotten area becomes hard and wooden. Some investigators have termed this symptom “hard rot,” but this term is not commonly used. Erwinia can survive in this dried out tissue for long periods of time.

Soft rot  on tubers, lesions can be as small as a single eye or involve the entire tuber. The rot is extremely soft and colorless. Although rot of the soft rot bacterium is relatively odor free, secondary organisms usually cause a foul smell.

Management and control of potato soft rot

To prevent soft rot do not over-irrigate fields during the growing potato season.
Harvest only mature tubers when soil temperatures are less than 10C. Minimize mechanical damage during harvest and handling.
Protect tubers from desiccation by sun and wind.

Cool tubers of early cultivars to 10C or lower as soon as possible after harvest then store them at 2 to 5C.  For late potatoes store tubers for 7-10 days at 10-15C to promote wound healing, then lower temperature to 2 – 5oC (7-10C for processing tubers).
Provide good ventilation to prevent low oxygen concentrations and development of moisture films on tuber surfaces.
Do not wash tubers before storage. If washing is necessary before marketing, dry the tubers as soon as possible and package them in well-aerated containers.
When washing use only clean water and change it frequently to reduce the soft rot inoculum level. Control other diseases that predispose tubers to soft rot.


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