Late blight is a disease which is common in solanaceous plants like potatoes, tomatoes, etc. It is caused by an oomycete pathogen, i.e., Phytophthora infestans. Late blight in a potato is a lethal disease which can have the potential to devastate the crop. It can attack the crop at any stage and can infect the foliage and tubers.
The initial symptoms are visible with small spots on the potato that appear circular and are light to dark green in colour. This is a destructive disease, which is why it becomes vital to understand the symptoms and manage them in the early stages.
P. infestans is a parasite which invades a host cell to live. It is a water mould which can affect the crop severely. They are coenocytic oomycetes. They can affect the crops by causing decay in the tubers. The quality and productivity of the crop also decrease when infected by P. infestans. It has the potential to destroy the crop completely.
Symptoms
As described earlier, the symptoms of this disease start with small circular spots that appear light to dark green in colour. These symptoms are visible on the leaves on the lower side, which gradually shifts towards the leaf tips or edges.
Some visible symptoms of the disease are listed below.
- Watery blisters of green or yellow colour appear on the leaf foliage.
- Brown or black lesions appear on the lower leaf.
- Leaf rotting after 1-4 days of infection; tan-coloured blisters appear in dry weather, while blue-grey mycelium grows in moist weather.
- Rotting in tubers, which are of two types: dry rot and wet rot.
- Dry rots are fungal growth which is 5-15 mm deep; the colour ranges from bluish-black to reddish-brown.
- Wet rots are deeper rots with 24-45 mm, which infect the entire potato. The colonies appear to be white, and they secrete water.
Apart from the above-mentioned symptoms, the weather also affects these lesions, and they are visible more during winters (cold and moist weather). They appear to be larger and dark brown in colour. As the infection increases, the blight kills the leaves in a few days. The lesions are not only limited to leaves but also move ahead to petioles and stems.
The late blight of potato can be confirmed by microscopic examination or by PCR. Sporulating samples that are collected from the affected leaves and tubers can be examined under a microscope. While for non-sporulating samples, PCR can be done to identify late blight genotypes.
Disease Cycle of Late Blight of Potato
The disease cycle of P. infestans, which causes the late blight in potato, occurs in two phases, i.e., the asexual and sexual phases.
Asexual Phase
This is more common as compared to the sexual mode of the disease cycle. Sporangia invade the plant with zoospores or by forming a germ tube. The biflagellate zoospores have survival potential in dry weather conditions. It can survive for 1 hour in dry conditions and even better in cloudy weather.
Zoospores (which fall from rain or are transmitted via wind) form a cyst around the host cell and infect it indirectly. The sporangia form a germ tube in dry weather while acting as an individual spore. It can directly invade the host and germinate.
Sexual Phase
This occurs via oospores which are formed as a result of karyogamy between antheridium (male) and oogonium (female). Followed by which the oospores develop sporangia by germination and follow the same pathway as described in the asexual mode of the disease cycle.
The pathogen has a very high potential of reproduction; it can produce up to 3 lakh sporangia in a single day from a lesion, followed by which every new sporangium targets a new host cell at different sites. As described earlier in the visible symptoms, these infections appear as small or wide patches on the stems and on the leaves.
Since the production of sporangia is really high, the growth of P. infestans is rapid. It causes the destruction of the tubers of potato crops and foliage too. It can rapidly grow to infest the entire field if managed inappropriately.
Late blight of potato, which is caused by P. infestans, is considered to be a community disease which can severely affect other crops too. It needs early diagnosis in order to manage it properly to avoid the devastation of affected crops and infection in other fields too.
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