More About Blight

Biology

The development of a Potato Blight epidemic is dependent upon certain weather criteria of temperature and humidity being achieved. This is measured as a Smith Period and occurs when 2 consecutive days have temperature recorded not less than 10°C and relative humidity above 90%. This must be for at least 11 hours of each day. Once a Smith Period is registered potato growers are advised to begin their potato blight fungicide programs.

The Potato Blight fungus can only survive in living plant material. This includes potato tubers in storage, infected potatoes missed during harvest and remaining unfrozen over winter (groundkeepers and volunteer potatoes), and on infected potato dumps. Transmission from infected plants to new foliage is primarily by airborne spores.

Potato Blight development is favoured by continual periods of high moisture and moderate temperature. Night temperatures of 10 – 15°C and day temperatures of 15 – 25°C are most conducive. Rain, dew or irrigation, keeping the relative humidity of the microclimate within the crop canopy above 90%, accelerates disease development. Potato Blight spores require water to germinate and penetrate the plant tissue.

Lesions on leaves and stems become visible as small flecks within a few days of infection. Theses lesions expand and appear as water-soaked, grey-green areas on infected leaves. The affected tissue becomes grey/tan in colour and very dry. Death follows within a few days. Lesions are often surrounded by a halo of lighter green tissue. Disease symptoms can also develop on leaf petioles and stem tissue.

Conditions must remain moist for a minimum of 7 to 10 hours for spore production to occur. This is why spores or lesions are most apparent after wet nights or periods of rainfall. There is often a white, mildew-like growth at the edge of the lesion, normally on the underside of the leaf. Spores are carried by wind and rain to healthy plant tissue, where the disease cycle begins again. The Potato Blight pathogen can complete many reproductive cycles in a season, accounting for the rapid increase in disease once it becomes established in a field.

Tubers are infected by spores washed from lesions to the soil. Tuber Blight infections are characterised by patches of brown to purple discolouration on the tuber skin. Cutting just below the skin reveals a dark, reddish-brown area of tissue that is dry and cork-like.

Product Choice

When choosing a fungicide to control Potato Blight three key features are consistently highlighted as being important. A reliable fungicide must have excellent efficacy, proven persistence, good protectant activity and be rainfast quickly. Potato Blight fungicides developed by Dow AgroSciences possess all these features. Other desirable features are ease of use and a level of curative activity.

Excellent protection from Potato Blight can be achieved using the extremely cost effective mancozeb fungicide Dithane* NT. Not only is the protectant performance of Dithane NT as good as many more expensive rivals, the formulation is the most technically advanced of any mancozeb fungicide available. Dithane NT is twice as rainfast as some other mancozeb formulations, with light periods of rain (no more than 15mm at a time) causing particles to redistribute across the leaf surface. This redistribution also takes place when morning dew forms on the leaf, increasing the protectant properties. Applications of Dithane NT should only be made to a dry leaf.

In common with all protectant fungicides, it is essential to apply Dithane NT before Blight appears in the crop. For this reason a regular spraying program should be planned. In the absence of a Blight warning, the first application should be made just before the potato haulm meets across the rows. If a Blight warning is issued the first application should be made immediately, irrespective of the growth stage of the crop. Further applications of Dithane NT can then be made to the potato crop every 7, 10 or 14 days, depending on local Blight risk and circumstances.

Tuber Blight is caused by twin-tailed zoospores that are produced from sporangia. They fall, or are washed down, from infected potato foliage and stems on to the soil. From here the zoospores swim in soil water and enter tubers via lenticels or cracks in the tuber wall. Zoospores are produced at lower temperatures, between 10 – 15°C, and are therefore common in the early part of the season and, most importantly, towards the latter end of the growing season.

Protection against Tuber Blight can be achieved either by killing zoospores after they are produced or by preventing their production in the first place.

When designing a Potato Blight fungicide program steps should be incorporated to minimise the risk of resistance. Resistance to fungicides can occur when a single site acting compound is used alone, especially if used repeatedly.  Mancozeb is a well known multi-site fungicide and consequently is the most common constituent in over two-thirds of all blight fungicides used in the UK.  It forms an essential component in any anti-resistance program. This robustness is reflected in the lack of restrictions on the product label.