Your tomato plants are growing outside. The plants are healthy and give lots of delicious fruit. Then around August, after a period of rainy weather, black and brown spots start appearing on the stems and leaves at first, but with in a week they spread quickly and the first tomatoes also start showing brown spots. If you are familiar with this, then you have Phytophthora in your tomato plants.

Phytophthora is a water mold that needs warm, moist conditions to develop on its hosts. Along with potatoes, unfortunately the tomato is also a host to this all-destroying fungus. Following the initial infection, it will spread very fast, especially in hot humid weather. I grow my tomatoes in a greenhouse and fortunately they never suffer from this mold, but if you grow tomato plants outside, then Phytophthora is your enemy number one.

As far as I know, there is nothing you can do when Phytophthora strikes your plants. The best thing is to ensure that the plants do not become contaminated in the first place. 

Don’t let tomato plants stay wet. Many people who grow tomatoes outside use transparent plastic to make a roof over the plants. This is often enough to avoid getting the Phytophthora fungus.

Prune more leaves by the end of July, so that the leaves can ventilate well and the plants will dry out fast after a shower. Read the article: “How remove old tomato leaves?”.

When you notice the first signs of Phytophthora infestation, immediately remove the infected parts. Keep doing this daily. Contamination starts long before it reveals itself through the brown spots.

Spraying milk helps a bit to make the infestation attack less quickly but only works preventively.

Add 0.5 liter skim milk to 0,5 liters of water and spray this regularly on the tomato plants (especially if a rainy period arrives).

If you grow outdoors and don’t want your tomato plants to rot in a wet summer because they have caught Phytophthora, you should grow Phytophthora-resistant varieties. That is the only thing that works well. Unfortunately there are only a few really good Phytophthora-resistant tomato varieties.

In Germany and Austria they have a classification system that gives you an idea of how resistant a particular tomato variety is.

2 - Good resistance: After affected parts are removed, little loss of yield.

3 - Adequate resistance: The plant is affected, but the harvest is still reasonable.

4 - Moderate resistance: The plant is badly affected, but does not die. The yield is limited.

Our 2 tomato varieties Farmzy® Little Red Tree and Farmzy® Sweet King are both very resistant to Phytophthora. They are both in Class 1 of the above classification.

Other varieties that are strong against Phytophthora are: F1 Damsol, F1 Juliet and Primabella.