Guide to tomato diseases caused by excess humidity

Over-irrigation is one of the big problems in agriculture, where it occurs much more frequently than lack of water.

This is why we wanted in this article to bring together the main tomato diseases due to excess moisturehow to identify them and possible treatments.

We have already foreseen that, generally, tomato diseases that have their origin in an excess of humidity (large amount of irrigation or ambient humidity) are difficult to removebecause they are usually related to highly resistant fungi and bacteria.


Main tomato diseases caused by excess humidity

Tomato late blight (Phytophthora infestans)

  • Causal agent: mushroom Phytophthora infestans.
  • Climatology: high relative humidity (above 90%) and temperature range 10-25 ºC.
  • Symptoms: aerial part of the plant at any stage of cultivation. Irregular brown spots appear which darken rapidly and affect all leaves and stems.
  • Damage: It affects the leaves, stems and fruits, especially the smaller ones, with brown spots located on the upper part of the fruits.

alternate (Alternaria Solani)

Causal agent: mushroom Alternaria Solani.Climatology: temperature range of 18-25°C and high ambient humidity.Symptoms: leaves and fruits, with small dark brown spots formed by concentric rings, surrounded by a yellowish circumference.Damage: dark, irregular leaf spots, often surrounded by yellow tissue. Black leathery spots on the fruit begin at the end of the stem. Sunken lesion of dark round stem with concentric circles inside.

Cladosporiosis (Passalora fulvapreviously fulvia fulva)

Causal agent: tomato cladosporiosis is caused by the fungus Passalora fulva (previously called fulvia fulva That is Cladosporium fulvum).Climatology: temperature range of 20-25 ºC and relative humidity conditions above 80-85%.Damage and symptoms:

  • Pale yellow spots on the upper side with grayish fruiting bodies on the underside.
  • Pale greenish-yellow spots, usually less than 1/4 inch in size, with no clear margins, form on the upper leaf surfaces.
  • A velvety olive-green to brown mold forms on the underside of the leaves below the leaf spots.
  • The leaf spots join together and turn brown. The leaves wilt and die, but often remain attached to the plant.
  • Old leaves are infected first.
  • Infected flowers turn black and fall off.
  • Fruit infections begin as a smooth, black, irregular area at the end of the fruit stem. As the disease progresses, the infected area becomes sunken, dry and leathery.

Botrytis, gray rot or gray mold (Botrytis cinerea)

Tomato BotrytisImage copyright: D. Blancard (INRA). Source:

Causal agent: mushroom Botrytis cinerea.Climatology: conditions of high relative humidity (> 80%) with a wide temperature range (optimum 17 to 23 ºC).Symptoms: large beige spots, wet and covered with gray felt.Damage: lesions on leaves, flowers and fruits. In the latter, it produces large brown spots that start from the top (calyx).

Discover here a complete guide in pictures of this tomato disease caused by excess humidity.

How to Differentiate Gray Mold From Other Problems

  • Botrytis cinerea it appears with cold temperatures and high humidity (more than 80%).
  • Under conditions of high humidity, gray, fluffy spores cover the infected parts of the plant.
  • The leaves have irregular V-shaped brown spots, often starting at the leaf margin.
  • Dieback symptoms appear as the infection progresses from the leaves, through the petioles, to the main stem.
  • Brown oval lesions may encircle the stem and infected flowers turn brown and die.
  • The spores are light grayish brown on black stalks.
  • Affected tomatoes develop a pale, soft, watery rot, or have white rings or halos on the fruit, called ghost spots.

gray spot (Stemphylium S. lycopersici)

Causal agent: mushroom Stemphylium S.lycopersici.Climatology: hot and humid weather.Symptoms: dotted with small angular brown spots.Damage: They cause severe damage to susceptible varieties, causing extensive defoliation.

Origin of excess moisture in tomatoes

Often, excess moisture in the tomato crop is not due to poor grower execution. If we talk about excess moisture in the soil, it can be due to poor irrigation management or a large amount of accumulated rain.

These problems usually arise in poorly drained soils, as clayey or heavy. And it is that solving a soil with excessive drainage (such as sandy soils), is relatively simple with the incorporation of organic matterat least in the planting hole.

However, improve drainage of heavy soils it’s a little more complicated, because a huge amount of sand has to be mixed in to improve water infiltration.

Water dose in tomato cultivation

Tomatoes planted outdoors in the spring and summer generally require between 1 and 4 liters of water per day.

  • Transplant until flowering: 1 L/plant every 2 days and, after 20 days, 1 L/plant every day
  • Fruiting and first fattening: 2-3 L/plant every day.
  • Fruit production and harvest: 3-4 L/plant every day.

If you want to delve deeper into this topic, we have prepared an article dedicated exclusively to tomato irrigation


Leave a Comment