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International fight against potato scab

Original article on FarmerWeeclyInteractive
New international work is under way to help potato growers assess the risk, before planting, of their crops getting powdery scab and rhizoctonia.

The Potato Council-funded £430,000 three-year study involves SCRI, SAC and the Food and Environment Research Agency (formerly CSL) with partners in Australia, New Zealand and South Africa.

It follows the success of earlier UK work which led to the DNA-based black dot prediction scheme now available as a commercial testing service.

VEGETABLE GARDEN Preplant InsecticideBlack dot is well suited to such diagnostic techniques, says the Potato Council’s Sue Cowgill. “The test will give you valuable information about how the disease will develop – whether you are looking for just an indication of how much of the pathogen is present or management advice based on the results.”

That advice was summarised in the council’s guide Managing the risk of black dot sent to levy-payers.

But getting to grips with other diseases, such as powdery scab and rhizoctonia, to allow similar reliable tests to be developed is a tougher challenge, which Dr Cowgill hopes the international collaboration will help overcome.

Predicting powdery scab is particularly tricky, she says.

“Both seed and soil-borne inoculum can cause the disease, but this isn’t the problem when it comes to determining its extent on progeny crops. Environmental conditions, such as wet summers, have much more influence.”

Forecasting rhizoctonia, which causes black scurf, is also far from straightforward in that it has developed on plants grown from disease-free mini-tubers even in soil where no inoculum has been detected.

“We think this comes down to how it’s distributed in the field. Its spread is probably patchier than black dot, which makes getting a representative sample more difficult.”

The international consortium aims to improve detection of such soil-borne pathogens through standardised testing, says colleague Mike Storey.

“We initially identified research synergies through discussions with our counterparts at Horticulture Australia.”

The non-competitive project has several benefits. “It’s a great opportunity to extend limited resources, gives a larger pool of research data, and we can share knowledge with leading international experts.

“By conducting trials in both the southern and northern hemisphere we can maximise seasons and study two crops a year, gaining data much faster.”

The research will evaluate the causes, distribution and control of soil-disease populations using the latest laboratory real-time PCR assays to test for pathogen presence, says Dr Storey.

“At the end of the study we hope to develop pre-planting tests that give you a quick, accurate assessment of disease risk of powdery scab and rhizoctonia in field conditions.”

The findings will be made available to levy-payers for disease risk assessment and decision-making purposes, along with disease control advice at the end of the study.


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Early blight potato disease

Early blightEarly blight potato disease is causes by  Alternaria solani fungal pathogen. Alternaria solani is generally thought to be a weak parasite.
Early blight is often a disease of senescence, where the older leaves are infected first. The disease can progress upward; attacking newer tissue as the older leaves droop and dry up. Under severe epidemics, leaves may be killed prematurely.
The disease first becomes evident in senescent leaves, in form of dark necrotic lesions in a characteristic concentric pattern. Contamination of the tubers is manifested by dry, dark, round depressions on the peel.
If cases of severe infestation it is possible loss of yield, due to early leaf death. Dry rot develops also on the tubers during storage.
Most of the early varieties are susceptible.

Management and control of early blight potato disease


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Use seed potatoes free of the disease. Seeds infected with the disease may damp off during germination.
Minimize stress by maintaining adequate soil moisture and fertility.      Avoid watering during the hottest hours and immediately after storms.
Early blight can be prevented with some fungicides: azoxystrobin, potassium bicarbonate, hydrogen dioxide as well as the biological control agent Bacillus subtilis
Avoid harvesting non-mature tubers. Protect against insect infestations.

Preventative measures include ensuring the healthy circulation of air in garden rows and rotating crops so that solanaceous plants are only present every three years, and choosing resistant cultivars.

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Dry rot potato disease

Dry rotSeveral species of Fusarium cause dry rot potato disease.
Young potato tubers have some resistance to dry rot which slows disease. But dry rot disease progresses noticeably during the last half of the storage season.

The first symptoms of dry rot are usually dark decay on the surface of the tuber. In cases of large lesions the skin becomes wrinkled in concentric rings as the underlying dead tissue desiccates. Internal symptoms are characterized by necrotic areas shaded from light to dark chocolate brown or black.
This necrotic tissue is usually dry and may develop at an injury such as a cut or bruise. The pathogen enters the tuber, often rotting out the center. Rotted cavities are often lined with mycelia and spores of various colors from yellow to white to pink depending on the species of the pathogen.

Management and control of dry rot potato disease

Practically the following procedures will help prevent dry rot.

  • Critical point is to purchase seed that has as little dry rot as possible. Seed should be inspected, preferably during the last months of storage.
  • To minimize injury and promote rapid growth Warm seed tubers should be warmed to at least 50° F before handling and cutting. Cold tubers are very prone to shatter bruising.
  • Clean and disinfect the seed storage before receiving seed. Knives on the cutter should be sharp to make a smooth cut for that easy healing.

Disinfect seed cutting and handling equipment often and clean up well between seed Plow Attachmentlots. Adjust the cutter and sort tubers to provide 2-ounce seed pieces that will provide substantial nutrition to the developing plant, even if some rot develops.
Get rid obviously rotted tubers before they reach the cutter.

  • Treating seed pieces with fungicide helps prevent dry rot and other diseases caused by seed pathogens.
  • Protect the seed tuber from affect of wind and sunlight during planting because dehydration greatly weakens them. Cutting pieces should be planted within 24 hours.
  • Harvest tubers after skins are set and when pulp temperature is greater than 50° F. Use antibruise practices when harvesting and piling potatoes.
  • If using a postharvest fungicide, be certain that the coverage is adequate to protect the entire surface of the tuber. Use the volume of water and fungicide rate specified on the label.
  • Allow a period for wounds to heal before dropping the temperature in storage. There should be good air circulation, high humidity (greater than 90%), and temperature around 55° F.
  • For prevention of condensation on the tubers drop the temperature slowly and store tubers as cool as possible, considering your intended market.
  • Monitor storages often for dry rot. Never place tubers with dry rot symptoms in storage because doing so spreads disease.