Late Blight

Phytophthora infestans is an oomycete, or water mould, a fungus-like microorganism that induces potato blight, or late blight, a severe potato and tomato disease. Alternaria solani causes early blight, which is also known as "potato blight." 

The 1840s European, 1845-1852 Irish, and 1846 Highland potato famines were all caused by late blight. Certain other members of the Solanaceae family may also be infected by the organism. Moisture and cooler temperatures are favourable to the pathogen: sporulation is best at 12–18 °C in water-saturated or almost saturated conditions, and zoospore development is best at temperatures under 15 °C (59 °F). At a relatively warm range of temperatures of 20 to 24 °C (68 to 75 °F), lesion growth rates are normally optimum.

Late blight of potato symptoms: Late blight symptoms include extreme yellowing, browning, spotting, withering, or death of leaves, flowers, berries, stems, or the entire plant.

Potato Blight Disease

Etymology:

Phytophthora is a genus that means "decay, fail, perish." The species name infestans is derived from the present participle of the Latin verb infestare, which means "to strike, kill," and wherein the word "infest" is derived. Heinrich Anton de Bary (1831–1888), a German mycologist, invented the term Phytophthora infestans in 1876.

Late Blight Diagram

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Disease Cycle of Late Blight of Potato

Phytophthora infestans does have an asexual life cycle that includes hyphal development, sporangia germination, sporulation, (either by zoospore production or direct germination, namely the germ tube emergence from the sporangium), and hyphal growth re-establishment.

When isolates of opposite mating types (A1 and A2) cross, there seems to be a sexual cycle. The development of sexual spores, known as oospores, is triggered by hormonal contact. The various forms of spores play an important role in the spread and survival of P. infestans. P. infestans may travel between host plants thanks to sporangia, which are dispersed by wind or water. The biflagellated and chemotactic zoospores produced by sporangia cause P. infestans to spread more on water films deposited on leaves or soils. Sporangia and zoospores are both short-lived, unlike oospores that can live for several years in a viable state.

The potato sign is white in colour. On the top of potato stems and leaves, Phytophthora infestans develops sporangia and sporangiophores. These sporangia and sporangiophores are often found on the foliage's lower surface. Tuber blight causes white mycelium to appear on the top of the tubers.

In perfect conditions, the life cycle of potato or tomato foliage could be completed in around five days. When temperatures are well above 10 °C (50 °F) and humidity is just over 75–80 percent for 2 days or more, sporangia grow on the leaves and is distributed throughout the crop. Rain may wash spores further into the soil, infecting young tubers, and spores could also be carried by the wind over vast distances. Blight in its early stages is easy to overlook. Dark blotches occur on the leaf tips and plant stems as a symptom. In humid conditions, white mould will emerge underneath the leaves, and the plant would collapse completely. Infected tubers grow grey or dark patches underneath the skin which are reddish-brown, and they rapidly degrade to a foul-smelling mush due to secondary soft bacterial rot infestation. When tubers appear to be healthy, they can rot later in the store.

Disease Management

P. infestans is a disease that is also difficult to eradicate. In agriculture, there are a variety of chemical choices for controlling harm to the foliage, fruit (for tomatoes), and tuber (for potatoes). Ridomil, a Gavel/SuperTin tank blend, and Previcur Flex are some of the most popular foliar-applied fungicides. Not only for resistance control but also because the potato plants would be targeted by several other pathogens at the very same time, all of the above fungicides must be tank combined with a broad-spectrum fungicide like mancozeb or chlorothalonil.

Resistant Plants: The difficulty in crossing cultivated potato to its wild relatives, which have been the potential source of resistance genes, have restricted the progress of resistance breeding, especially in potato plants. Furthermore, most resistance genes only function against a subset of P. infestans isolates since successful plant disease resistance is only achieved whenever the pathogen produces an RXLR effector gene that matches the corresponding plant resistance (R) gene; effector-R gene interactions activate a variety of plant defences, including the development of pathogen-toxic compounds.

Reducing Inoculum: The source of inoculum could be limited to control potato blight. Planting can only be done with high-quality seed potatoes and tomatoes from reputable sources. Inoculum could be found in leftover potatoes from the following years and self-sown tubers.

Heat can be used to destroy oomycetes including Phytophthora infestans in soil, compost, or potting medium. Oomycetes should be sterilised at 120F(49C) for 30 minutes.

Environmental Conditions: P. infestans thrives in a number of different environments. During the 2009 growing season throughout the United States, an instance of this occurred. Because the weather was cooler than usual for the season and there was more rain than usual, there had been a significant infestation of tomato plants, especially in the eastern states.

If the following requirements arise as the crop canopy closes, then fungicides should be used to avoid an outbreak, according to weather forecasting systems like BLITECAST.

• A Beaumont Period is described as a time of 48 hours wherein the hourly temperature and relative humidity readings at a particular location are not less than 10 degrees Celsius (50 degrees Fahrenheit) and 75%, accordingly.

• A Smith Period is described as at least every two days with a minimum temperature of 10 °C (50 °F) or higher as well as at least 11 hours of relative humidity higher than 90% on every day.

Use of Fungicides: Fungicides for the prevention of late blight of potato are usually always used as a preventative measure, with disease forecasting like an alternative. Fungicide applications can be required weekly in susceptible cultivars. The most powerful spray is one that is applied early in the day. The type of fungicide used can be determined by the existence of local P. infestans strains.  Metalaxyl is a fungicide that has been marketed to be used against P. infestans, but when used alone, it causes severe resistance issues. During the 1980s and 1990s, numerous strains of P. infestans were immune to metalaxyl in some parts of the world, however, most populations reverted to vulnerability in subsequent years.

Control of Tuber Blight: Ridging is sometimes used to prevent blight contamination of tubers. This usually entails piling soil or mulch all around stems of the potato blight, causing the pathogen to move a longer distance to reach the tuber. Another choice is to burn off the vegetation in the canopy for five weeks until harvest with a touch herbicide or sulfuric acid.