the water mold that causes late blight in potatoes

Late blight is caused by a pathogen such as the fungal oomycete Phytophthora infestans. The main host is the potato, but P. infestans can infect other solanaceous plants, including tomatoes, petunias and hairy nightshade. These infected species can serve as a source of inoculum for potatoes.

Late blight is caused by a pathogen such as the fungal oomycete Phytophthora infestans. The main host is the potato, but P. infestans can infect other solanaceous plants, including tomatoes, petunias and hairy nightshade. These infected species can serve as a source of inoculum for potatoes.

In North America, late blight survives among seasons on infected seed tubers, cull piles and volunteer plants. This destructive disease can infect potato leaves and tubers at any time of the crop's growth. Knowing the symptoms and illness cycle of this rapidly destructive illness is necessary to implement management strategies.

SYMPTOMS

1. The first signs of late formation in the fields are small, light to dark green, circular to watery spots of irregular shape. These lesions usually appear first on the minor leaves. Lesions usually start near the leaf tips or edges, where the dew stays the longest.
2. During cool, wet weather, these lesions develop rapidly into large, brown or black lesions, often greasy. 
3. A yellow chlorotic halo often surrounds leaf lesions.
4. Lesions are not limited to the veins of the leaflets, and when new infections occur and existing infections combine, the leaves can become damaged and die in just a few days. Lesions may be present on the petioles and stems of the plant.
5. During active increase, especially in cool, wet weather, an area with white mould is visible at the end of the lesions or near the petioles. 
6. This is the place where the late tuberculosis virus produces spores. When the climate becomes warmer and drier, these lesions dry up, stop sporulating and turn black. A yellow-green border often surrounds the lesions. Heavily infected fields often produce a distinctive odour. 
7. Late infection of the tubers is characterized by irregular, slightly depressed areas that can vary greatly in size and colour from brown to purplish on the skin. These symptoms may be less noticeable in some russet and red-skinned breeds.
8. Brown to red-brown, dry, granular rot is found under the skin in brown areas and penetrates the tuber usually less than ½ inch. The degree of tuber rot depends on the tenderness of the tuber, the temperature and the length of time after the initial infection.
9. The proportion of diseased cells is not always distinguishable, but it can be, especially in seed potatoes kept at cold storage temperatures. However, the line between blight-infected and healthy tissue is often marked by brown finger-like enlargements in the healthy tuber flesh.

Infections with other tuber rot pathogens, such as bacterial soft rot, pink rot or frequent leaks, can use blight-affected areas over time as infection centres and often progress rapidly. Then P. infestans, making diagnosis difficult.

The microscopic examination of samples from infected leaves or tubers can correct knowledge of late blight. PCR can do identification of intact samples and identification of late TB. 

Some late blight pathogens have been documented to have resistance to the fungicide mefenoxam/metalaxyl. Identifying the type is important to ensure effective chemical treatment when late blight is confirmed. Contact your local Extension office for present information on the latest fungicide-resistance genes.

DISEASE CYCLE

P. infestans, the reason for late blight, is a heterothallic fungal-like pathogen, which means that two mating types are required for reproduction and are called A1 and A2. The pathogen requires live plant or tuber tissue to survive in the field without oospores.

However, in some parts of the world where there are two types of the virus, a sexual cover is produced that can live in the soil. Although soil survival is not known to happen clearly in the US, anecdotal evidence indicates that sexual mixing has occurred, suggesting that soil survival is possible.

Infected tubers are the main source of inoculum for P. infestans. That includes stored potatoes, infected tubers missed at harvest that stay unfrozen during the winter (volunteers), infected seed tubers and cull piles, and P. infestans other host plants. The pathogen can be transferred from infected seed tubers to newly emerged potato plants, producing airborne pathogens that can migrate to neighbouring plants.

The late blight pathogen is favoured by free moisture and cool to moderate temperatures. Nighttime temperatures of 50 to 60 F and daytime temperatures of 60 to 70 F are best for disease development. Free rainwater, dew and surface irrigation water provide the water needed for infection and pathogen development.

Spores emerge in three to five days, requiring 12 hours of free wetness for infection. The leaves and stems appear as small blisters a few days after infection.

Lesions extend to water-soaked, grey-green areas on the leaf and are destructive if conditions are favourable. The spores are carried by air and rain to healthy plants, where the cycle of death begins again. The death cycle can occur every five to seven days, which causes the rapid spread and movement of late blight.

The tubers are infected by spores washed from the lesions into the soil. Spores germinate and swim on tubers in free water and are highly infectious to the eyes. Tuber pathogens appear as brown to purple lesions on the potato skin. Cutting under the skin reveals dark, reddish-brown, dry, and corky rot.

MANAGEMENT

Effective control of the disease requires the implementation of an integrated approach to disease control. Back pain is a social disease, and effective control requires social control. Here are some ways to help manage the disease:

• Destroy all cull and volunteer potatoes.
• Plant late blight-free tubers.
• Do not mix seed pods because cuttings can transmit late blight.
• Use a seed coat with a fungicide treatment labelled for late blight control (Who can find a current list of fungicides in the North Dakota Field Crop Plant Disease Management Guide, PP622). Recommended seed treatments include Revus, Reason and mancozeb.
• Avoid planting in problem areas that may stay wet for a long time or may be difficult to spray (field near the centre of the pivot, near power lines and tree lines).
• Avoid excessive and night watering.
• Eliminate inoculum sources such as hairy nightshade weed species and volunteer potatoes.
• Spray fields regularly, especially in low, wet areas, near tree lines, mid-pivot and other areas that stay wet for long periods where late blight may occur.
• Apply foliar fungicides on a regular and continuous schedule. Once late blight is present, only foliar fungicides can control late blight in the field. (Who can find a current list of fungicides in the NDSU "Fungicide Guide," PP622)?
• Stay tuned for the latest blight estimates. In our area, the NDSU Potato Blightline (https://ndawn.ndsu.nodak.edu/potato-late-blight.html) is active during the growing season and provides weekly updates and forecasts.
• Quickly destroy hot spots of late blight.
• Kill the vines completely two to three weeks before harvest. Consider adding a fungicide when the vines are dead if there is recent blight pressure.
• Applying phosphorus to potatoes after harvest and before harvesting can prevent late blight infection and spread during storage.
• Check your local home garden and tomato market for the latest root cause. Late blight can spread from these local sources to potato fields.