Late blight is a fungal disease caused by Phytophthora infestans fungus, which is primarily widespread in wet weather – the greatest risk of spreading diseases is during frequent rains in the summer. Only in conditions of too much moisture a massive spread of this disease can occur, and mass decay of crops. This happens very quickly – within 1 week the entire plant can be destroyed.
It is interesting that the onset of this disease is usually common during the season on a potato, and later it transfers to tomatoes. The initial appearance of the disease is visible on the back of the lower leaves as gray slices, in the later stage it appears on the plants as a brown rot: it destroys leaves that look burnt on the leaves and have yellow and brown stains on the leaf surface, later browning and decaying.
During the wet weather, a gray-white coating can be clearly seen on the lower leaves. Late blight affects the family of plants such as tomatoes and potatoes. Once the leaves are affected, it slowly moves to other parts of the plant from the leaves.
You noticed late blight! Now What….
As soon as you notice the disease, immediately remove and burn the infected parts to prevent the spread of the disease. If you water the tomatoes, do not water directly on the plants, water at the end of the stem – on the ground. Also, tightening the tomatoes with stalks and moving the lower leaves also prevents moisture retention and stops the spread of the disease – because then where the most moisture is retained – at the bottom of the soil, it is much more airy.
You should be very careful, check regularly for signs of this disease if the weather is too wet. In such conditions it is virtually impossible to stop the spread of the disease, plants and fruits will decay right before your eyes. If you spot signs, try to harvest what you can, at least to save some fruits.
Cherry tomatoes are the most resilient. You should try growing these if you have wet weather in your area.
How To Prevent Late Blight On Tomatoes
Prevent overwintering: Fortunately, the fungus that causes late blight needs living tissue to survive over the winter, so it can’t overwinter on tomato cages or supports. However, infected potatoes (the other plant that gets late blight) can carry the disease through the winter. Be sure to destroy any volunteer potato plants that come up. If you plant potatoes again, be sure to buy seed potatoes that are certified as disease-free.
Give plants space: If possible, avoid planting tomatoes and potatoes where you had them last year. Be sure to give plants plenty of space, based on recommendations for the variety. Maximizing airflow and light around the plants will help them resist disease. Make use of trellises and supports that will keep the vines off the ground.
Avoid watering from above: Using soaker hoses or drip irrigation keep foliage dry, which makes it more difficult for late blight — and other diseases — to spread. Avoid overhead watering techniques (sprinklers). Water early in the day so the foliage can dry before nightfall.
Spray preventively, if necessary: The key word is preventively. Once plants are infected with late blight, it’s too late to save them. Research is ongoing in determining the best organic options. Cornell plant pathologist Meg McGrath suggests using Actinovate(which contains the beneficial bacteria Streptomyces lydicus) as a preventative spray, and adding a copper-based product, such as Copper Fungicide, when late blight is present. Before using any products, read the label and use them accordingly. In most cases, effective protection requires that plants be sprayed as often as weekly throughout the growing season. Remember that while these sprays can help reduce the likelihood of infection, you still need to monitor plants closely.
There are many fungal diseases of tomato and they all seem pretty similar, so it is very important to know the symptoms and also to know what weather conditions will spread a particular disease. Late blight is frequent at very high humidity, only then it is spreading rapidly and causing more damage to tomatoes. If you have brown spots, stains, and dry leaves, it does not necessarily mean that you are affected by late blight.