Phytophthora infestans (late blight)

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Phytophthora infestans (late blight)

Late blight is a fungal disease which considered as one of the important diseases that affect tomato and potato crops and causes them significant economic losses. Its danger stem from its rapid spread.


Symptoms of infection on tomato plants are characterized by the appearance of dead spaces on the edges of the leaves, then they expand to cover the entire surface of the leaves then their color turns into black. Symptoms of late blight appear on all plant parts, leaves, stems. and petioles. Moreover, the infection appears in the form of brown flats connected to each other of dying tissues.


On the leaves, there appear irregular shape spots of black color that tends to reddish brown, watery that looks like boiled and surrounded by a pale area and these spots are found until it covers the whole surface of the leaves when it turns black.

If the air humidity increases as a result of rain or a lot of dew, a white fluff appears on the lower surface near the edges of the spots, which are microbial carriers and sporangia sori of the causative fungus. As the infestation intensifies, the leaves will fall down, but if the weather is dry, the spots will not expand, its colour will turn brown, and become easy to be broken.


On the stems, spots appear similar to those affecting the leaves. The infestation extends from the top of the plant to the bottom. The spots extend around the stem, where they dry and crack longitudinally and become easy to break.

Tomato fruits are sensitive to infection at all its growth stages, the symptoms often start on the fruits at the neck area, where not bordered shape spots with gray to green colour appear on ripe and semi-ripe fruits.

As for the infection on green fruits, it appears in the form of dark brown spots with a tough structure and marked edges.

Life cycle

The fungus is transmitted by the sporangia that separates from the carrier easily and is carried by airflows. The sporangia sacs either sprouts directly and gives a germination tube, at the end of which a terminal sporophyte sac arises, or indirectly where the ciliated spores germinate, causing the infection.


Suitable conditions for the occurrence and development of the disease:

  • Humid nights that tend to cool
  • High temperature during the day with the continual high humidity.
  • Entity of dew during the night for a period of not less than four hours.
  • Low humidity with temperature above 24C° limits the growth of fungi and the spread of disease.
  • Spores lose their vitality when humidity drops below 50%


The early warning system to forecast the occurrence of the disease can be briefed on two conditions:

  • Minimum temperature for two consecutive days of not less than 10°C
  • Relative humidity during the same two days is more than 75%


It is necessary to spray with preventive pesticides when forecasting the infection because tomato and potato plants get the disease when the appropriate environmental conditions are available, and therefore it is necessary to spray continuously with chemical pesticides every 15 days to protect the plants from infection and we should not wait until the infection develops, then spraying will not be effective.

Pest Management Methods

  • Do not plant plants in dense in order to prevent increasing humidity inside and between them.
  • Plants can be protected before infection occurs by spraying chemical pesticides such as: Mancozeb – Copper compounds such as: Copper Hydroxide, Copper Oxychloride or organic copper like Scudo


Plants treated with dilute copper solutions, especially organic copper, like Scudo, remain green for a longer period, as copper stabilizes chlorophyll and slows down the physiological aging processes of the plastids and prolongs the leaf activity period. It also activates respiration processes and increases the speed of oxidation processes in the plant.


When the infection appears, the following pesticides can be sprayed:

Flibocolide – Propamocarb hydrochloride – Cymoxanil – Metalaxyl – Benalaxyl – Fosetyl aluminum – Azoxystrobin.

Eng.Yamen Ahmed

Agrotica Debbaneagri

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