Potato seed infected late in the season with Dickeya (new blackleg) usually does not show symptoms in the field before harvest nor in seed storages. This is because Dickeya requires high temperatures for the development of visible symptoms. The optimum temperature for Dickeya is above 25 C.
By contrast, the old blackleg (Pectobacterium) can develop at cool temperatures (8 C to 10 C), and symptoms are usually visible when cutting seed.
Banks says she received two questions about Dickeya:
Q: If dormant infection of Dickeya is suspected, could you incubate a sample of tubers at 25 C to 30 C so the tubers will show Dickeya symptoms in about 2 weeks?
A: Banks asked Steve Johnson (Maine) and Gary Secor (North Dakota) this question and both said no. It takes more than two weeks for the symptoms to develop. Banks says the first time she saw Dickeya symptoms developing from seed tubers with dormant infection was in late June (the seed tubers had been planted by the middle of May), and both Johnson and Secor agree.
Q: Are Dickeya lesions smelly?
A: We all know how smelly the old blackleg is. The slimy, black stems smell like rotten fish, a disgusting smell noticeable at least 30 feet away from an infected plant. Usually Dickeya is not smelly. The bacterium grows inside the stem moving up in the vascular tissue. Dickeya-infected stems are usually dark brown, not inky black, slimy and smelly like the old blackleg. Eventually, the infected stems may be invaded by secondary bacteria that cause a black rot. If, shortly after emergence, you see small, wilted plants with a black stem base, it is probably the old blackleg. If the summer is cool and wet, probably the old blackleg will prevail. In hot summers, Dickeya will be the prevalent disease (wilted foliage is also a symptom of Dickeya).
The field trial conducted by The Sainsbury Laboratory (TSL) in Norwich involves incorporating late blight resistant genes from a wild potato relative into a cultivated Maris Piper potato. READ MORE
An infected potato psyllid insect carries the Lso (Candidatus Liberibacter solanacearum) pathogen that can cause zebra chip disease in potato crops.
Zebra chip has affected potato crops in the U.S., Mexico and New Zealand and caused millions of dollars in losses. Potatoes with zebra chip develop unsightly dark lines when fried, making affected potatoes unsellable.
The first detection of Lso came from sampling cards collected at one site south of Highway 3, near Lethbridge, Alta. For the full story, click here.
Harvest management, in large part, is bruise management. Bruising also affects tuber quality significantly. In order to harvest potatoes with minimum tuber damage, growers need to implement digging, handling and storage management practices that maintain the crop quality for as long as possible after harvest.
Assuming all harvest and handling equipment are mechanically ready to harvest the crop with minimum bruising, there are several tips to preserve the quality of potatoes crop during harvest:
- Timely Vine Killing. Killing the vines when tubers are mature makes harvesting easier by reducing the total vine mass moving through the harvester. This allows an easier separation of tubers from vines.
- Timely Harvest. Potatoes intended for long term storage should not be harvested until the vines have been dead for at least 14 days to allow for full skin set to occur.
- Soil Moisture. Optimal harvest conditions are at 60-65% available soil moisture.
- Tuber Pulp Temperature. Optimal pulp temperatures for harvest are from 500F to 600F. Proper pulp temperature is critical; tubers are very sensitive to bruising when the pulp temperature is below 450F. If pulp temperatures are above 650F, tubers become very susceptible to soft rot and Pythium leak. Pulp temperatures above 70°F increase the risk of pink rot tremendously no matter how gently you handle the tubers if there is inoculum in the soil.
- Tuber Hydration. An intermediate level of tuber hydration results in the least bruising. Overhydrated tubers dug from wet soil are highly sensitive to shatter bruising especially when the pulp temperature is below 450F. In addition, tubers harvested from cold, wet soil are more difficult to cure and more prone to breakdown in storage. Slightly dehydrated tubers dug from dry soil are highly sensitive to blackspot bruising.
- Reducing Blackspot Bruising. Irrigate soil that is excessively dry before digging to prevent tuber dehydration and blackspot bruising.
- Bruise Detection Devices. Try to keep the volume of soil and tubers moving through the digger at capacity at all points of the machine. If bruising is noticeable, use a bruise detection device to determine where in the machinery the tubers are being bruised.
- Do not harvest potatoes from low, poorly drained areas of a field where water may have accumulated and/or dig tests have indicated the presence of tubers infected with late blight.
- Train all employees on how to reduce bruising. Harvester operators must be continually on the lookout for equipment problems that may be damaging tubers. Ideally, growers should implement a bruise management program that includes all aspects of potato production from planting through harvest.
- Harvest when day temperatures are not too warm to avoid tuber infections. Storage rots develop very rapidly at high temperatures and spread easily in storage. If potatoes are harvested at temperatures above 27o C and cool off slowly in storage, the likelihood of storage rots is increased. If warm weather is forecast, dig the crop early in the morning when it is not so warm.
The warm conditions has slowed the harvest of some processing fields, to prevent bringing warm tubers into storage. The seasonal accumulated precipitation has been 50-70% of normal in the potato growing areas (Fig1). The soils are generally on the dry side (Fig 2), but irrigated fields have sufficient moisture for a good harvest. READ MORE
What is late blight?
Late blight is a disease caused by an organism that produces a white fuzz on the underside of leaves which releases millions of spores that float through the air to infect other plants. The spores land on a susceptible leaf, germinate, and cause brown oily lesions. The spores splash on the ground and infect potato tubers, which become brown and rusty looking, with a granular texture. Crop losses due to late blight can cost the Canadian potato industry tens of millions of dollars annually.
Protecting the potato industry
AAFC late blight specialist Rick Peters says taking steps to prevent the disease from infecting potato crops is important to help protect the health of the industry. He advises home gardeners to ensure their tomato seeds are resistant to the US-23 strain of late blight. Resistant seeds can be purchased at most garden centres. Certified disease-free seed potatoes can also be found at garden centres or purchased from a local seed potato grower. Peters says potatoes grown from last year’s garden or those bought from the grocery store are not suitable for planting as these tubers have not been tested and certified as disease-free and could be susceptible to a variety of potato diseases.
AAFC has partnered with industry leaders to identify and track late blight strains in production areas across the country. Scientists are also looking at biological characteristics of the different strains including how they respond to treatments. This knowledge allows for better management and control of the strains in Canadian potato and tomato production areas. While scientists continue to study the disease, they maintain that an ounce of prevention is worth a pound of cure and home gardeners have an important role to play.
If you spot a suspected late blight infection in your garden this season, please contact the Department of Agriculture, Aquaculture and Fisheries at 1-866-778-3762 for information on how to properly dispose of infected plants.
Sept. 29, 2016 – Second growth is a physiological potato problem induced by soil temperatures of 24 C or above and water stress. These two factors interact to limit the tuber growth rate, causing second growth. Inadequate soil moisture alone does not result in the initiation of second growth.
Heat and drought prevailed during the 2016 Ontario growing season, which explains why second growth has been reported in some fields.
Potato varieties differ in their susceptibility to second growth. European varieties appear to be more susceptible because they were bred and evaluated in countries where the growing seasons are rarely hot.
There are three distinct types of second growth:
Tuber chaining: A series of small tubers are produced on a single stolon.
Heat sprouts: Sprouts develop from stolons or daughter tubers. The sprouts may emerge from the hills developing into leafy stems.
Secondary Tuber: Small tubers form on daughter tubers. The secondary tubers are formed on short sprouts or directly on the tuber surface. This disorder is usually associated with physiologically old potatoes. High temperatures and water stress during the growing season are major factors contributing to the physiological aging of potatoes.
Cultural practices that promote uniform growth of plants and tubers throughout the season help minimize second growth. Among them are:
● Do not plant physiologically old seed in cold, dry soil.
● Space seed pieces as uniformly as possible at planting.
● Apply an adequate amount of fertilizers.
● Maintain uniform soil moisture sufficient to meet crop needs (this was easier said than done this past season!).
Sept. 8, 2016 - Although harvest of the late maturing crop has not started yet, it is never too early to start thinking about disease management in 2017. Two fungicides to keep in mind in 2017 are QUASH for early blight and Revus as a seed treatment for late blight.
Sept. 8, 2016 - According to Manitoba Agriculture, aphid counts in weeks 9 increased slightly in most locations. However, one western field had no aphid trapped. While another field in the same region continued to have massive numbers; with significantly higher potato aphids compared to last week. Most of the seed fields are being desiccated, so this will be the last week of aphid report.
One more potato psyllid adult was confirmed on Aug. 24 in a card from Northfolk-Treherne Rural Municipality.For more information and detailed report please visit: www.mbpotatoes.ca
August 15, 2016 - New late blight finds were reported on potato from Carman, Winkler areas and on tomato crops east of East of Portage and east of Highway #75, according to Vikram Bisht. Frequent fungicide applications are being applied to control the disease; and in one case the tomato plants have been pulled out and destroyed.
Samples have been collected for strain identification. All of the previous samples, tested by Lethbridge Research and Development Center were determined to be US-23 strain.
"There is increasing metalaxyl insensitivity in the Pi from these samples and the use of Ridomil would probably have only marginal benefit," says Bisht.
It is important, he continues, to scout for late blight, especially in low lying, irrigation pivot center, wheel tracks of irrigation systems (guns/pivots), tree-line protected areas and under hydro-power lines (areas where applicators may have difficulty covering).
It is also critical at this time to monitor potato and tomato plants in home gardens. The DSVs (late blight risk values) accumulated over 7-days at various weather stations suggest mostly moderate risk in most of the province. There is forecast for rain and risk of thunderstorms today afternoon in many potato growing areas. Full fungicide coverage of foliage in high risk areas should be maintained.
Due to continued high moisture levels in many fields, it may be helpful to harvest the low lying areas last, so it will be easier to manage the storages.
Also, a post-harvest treatment with phosphorus acid / phosphite fungicide could be considered for such fields, adds Bisht.
The number of new finds of late blight seems to have slowed down, even though the disease continues to be a concern. All of the isolates tested so far, were determined to be US23. Late blight was found in market-garden plots of potato and tomato in Oakville area, in Central Manitoba.
There were scattered rains and strong winds on Aug. 4 and 7, which may have spread the disease.
It is extremely important to continue to scout for late blight, especially in low lying, irrigation pivot center, wheel tracks of irrigation systems (guns/pivots), tree-line protected areas and under hydro-power lines (areas where applicators may have difficulty covering). Full fungicide coverage of foliage in high risk areas should be maintained. It is also critical at this time to monitor potato and tomato plants in home gardens.
The DSVs (late blight risk values) accumulated over seven days at various weather stations suggest mostly moderate risk in most of the province. There is forecast for a few rain days in many potato growing areas, in the coming week.
Due to wet and warm conditions there are reports of stem rot/blackleg. Hail damage and European Corn Borer (ECB) injury appears to have contributed to some of the stem rotting. Early blight in general appears to be very minor.
The aphid counts remained low in the third week (July 5-11) and the fourth week (July 12-18) especially in the southern seed production area. Potato aphids, but not Green Peach aphids (GPAs), were found in these weeks. There were no aster leafhoppers (ALH) and potato leafhoppers (PLH) noted in the traps.
In the fifth week (July 19-25), the aphid counts have increased significantly over the previous week. Green peach aphids were trapped from the Portage area only. The potato aphids were trapped in all the three seed production areas. Potato aphids are fairly efficient PVY transmitters, but not as efficient as GPAs. The “other aphids” in the traps are poor transmitters, but make up with higher numbers.
With other crops in the region maturing and near harvest, the aphids will find the green potato crop very attractive. It may be helpful to the seed growers to consider tank mixing insecticide with the aphid-oils application, especially if the crop planted had some level of PVY in the seed itself.
The results from suction and pan traps in seed fields for the third, fourth and fifth week can be found here.
Currently, there is no report of any serious Colorado potato beetle (CPB) feeding in commercial potatoes.
European Corn Borer:
Delta trap monitoring for the ECB moths using pheromone lures continue to show some adult moth activity – in Carberry, Brookdale in Rural Municipality of North Cypress-Langford, Treherne (RM of Victoria), Shilo (RM Cornwallis), Glenboro (RM of Glenboro-South Cypress) and Carman (RM of Dufferin) area.
After a peak activity in mid-July, the number of trapping has reduced. After the appearance of very young larvae (Figure 1) was the trigger for insecticide application in fields close to last year’s serious infestations. Some ECB injury and larvae were noticed in the Carberry area.
Heat edema has been observed in Ontario, and should disappear when humidex values drop below 30 C. Photo courtesy of Eugenia Banks.
July 27, 2016, Ontario – Two potato problems have developed due to high temperatures, writes Eugenia Banks in her latest Ontario potato update.
The long and relentless heat wave that is affecting Ontario has provided favorable conditions for diseases and physiological problems that we do not often see. One of them is Fusarium wilt caused by Fusarium oxysporum, a relatively common soil fungus that thrives when soil temperatures are around 29 C.
A grower found patches of Fusarium wilt this year in a field of Andovers. Also wilted plants here and there were noticeable in the field. The base of the stems emerging from the seed was woody. At ground level the stems were dark brown, rotten or hollowed but did not have a strong fishy smell. The upper part of the stems was still green but wilted. Most of the plants pulled had rotten seed, but with no fishy smell.
Gary Secor from North Dakota State University helped with the identification of this problem. He isolated Fusarium oxysporum from the diseased stems. There was no sign of blackleg, dickeya or verticillium.
How to control this disease?
Fusarium oxysporum is more prevalent in very hot summers, just like our 2016 growing season. There is nothing that can be done about the weather, but:
● Avoid ammonium nitrogen as a source of N
● Keep potassium levels adequate
● Acid soils favour Fusarium wilt. Liming soils help, but may increase the risk of common scab.
This problem has been observed in two fields of the variety Canela Russet, growing in two different production areas. Small bumps form on the leaves that, with time, rupture leaving round, brown necrotic spots. The centres of the spots often drop out leaving holes in the leaves. Holes in leaves usually means insect feeding, but there were no insects in the fields. Ian MacRae, an entomologist at the University of Minnesota, did not think that the holes looked like insect feeding. Jeff Miller from Miller Research in Idaho suggested heat edema, a physiological disorder. Edema is related to water retention (see page 122 in the Potato Field Guide), but heat edema is different. The cells expand trying to diffuse the heat and eventually rupture. Because it is a physiological problem, heat edema should disappear when humidex values (temperature plus humidity) drop below 30 C. Yesterday, I checked one of the fields and noticed that, because of the hot weather, necrotic spots were developing on new leaves.
Prolonged heat waves bring all sorts of different problems to potato plants. Working with potatoes is a never-ending learning experience!
July 27, 2016, Manitoba – Several fields in western Manitoba (Carberry area) reportedly have light blight symptoms, according to Vikram Bisht's latest potato disease reports.
Some photographs of the diseased leaves show symptoms typical of late blight. Three samples received have been confirmed to have sporangia. The infections are mostly on the top foliage and found in pivot wheel tracks, along the irrigation gun run and edge of field in low areas. There are very few infected plants in the fields and they are difficult to find. The five-day spray schedule in these high risk fields may have kept the infection low.
So far, there is no report of late blight from any other part of Manitoba. The DSVs (late blight risk values) accumulated over seven days at various weather stations continue to suggest high late blight risk in most of the province.
It is extremely important to scout for late blight now, especially in low lying, irrigation pivot center, wheel tracks of irrigation systems (guns/pivots) and tree-line protected areas. It is also critical at this time to monitor potato and tomato plants in home gardens.
There is no forecast for rain events for rest of the week, but it will be warm.
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