Control late blight, Phytophthora infestans on tomatoes and potatoes

The causative agent of late blight is an egg fungus. This has specialized in the infestation of nightshades, especially potatoes and tomatoes. It rarely chooses aubergines or peppers as host plants. Since the spores are still causing sensitive yield losses today, armies of scientists and gardeners are dealing with the problem, so that an extensive wealth of experience has now been gathered to combat it effectively. Although the range of fungicides used in the home garden is very limited, a number of effective procedures have emerged on the part of health and environmentally conscious methods from which hobby gardeners benefit. Without slipping into scientific sophistry,

• The late blight fungus overwinters in infected potato tubers and accumulated crop residues
• Activity is triggered by temperatures of 15-23 degrees Celsius and humidity
• If the leaf moisture persists for more than 10 hours, the spread begins
• The main way of spreading is by wind and irrigation water
• Infestation of the host plants takes place via both injured and intact tissue
• Period of development of a spore generation is 3-4 days
• A mature fungus survives drought periods for several weeks in host plants

What makes the brown rot fungus so dangerous is its ability to flexibly adapt to any control method through mutation. There is therefore a permanent race between humans and pathogens. A clear winner has not yet emerged.

Damage to potatoes

If June is characterized by warm, humid weather, grey-green spots appear on the leaves, which subsequently turn brown and spread to the stems. At the same time, the leaves begin to curl up. If you now take a look at the underside of the leaves, you will see a grey-white mold lawn there. Now it won’t be long before the fungus reaches the tubers. Sunken spots appear, under which the flesh turns red-brown. If you cut open a potato, the whole catastrophe is revealed to you in the form of rotting down to the middle of the tuber. In the case of potatoes in harvest storage, soft rot bacteria are often added. It follows that not only stored potatoes, but also the seed can be affected by late blight or brown rot.

preventive measures

Late blight only poses a risk if the summer is characterized by frequent rainfall. Since permanently dry weather in the latitudes here is rather the exception, preventive measures should be considered in advance of potato cultivation in order to at least keep the fungal infection under control in an emergency.

• Avoid varieties that have been proven to be susceptible in favor of resistant varieties such as Christa or Nicola
• Plant the potato bed far away from the tomato bed
• Choose a generous planting distance for the most airy conditions possible
• Align the seed rows according to the prevailing wind direction from West to East
• Immunize harvest potatoes to be used as seed for 1 week in the sun
• Soak seed tubers in horsetail broth for a quarter of an hour before planting
• Dust the bed and the potato plants weekly with rock dust
• Mulch the soil regularly to avoid splashing water
• The plants strengthen with rhubarb tea, horsetail tea or nettle stock
• Growing potatoes in a mixed culture with celery, hemp or beans
• Administer strengthening preparations, such as liverwort extract or Ulmasud B
• Instead of using nitrogen-rich fertilizers, it is better to fertilize with comfrey manure

If the potato cultivation went smoothly, an experienced hobby gardener will not let up during the harvest. In view of the next season, the following behaviors also serve to prevent Phytophthora infestans:

• First rip out and destroy the potato cabbage
• Always harvest in warm, dry weather
• Never wash harvested potatoes before storing them

If dry, warm weather prevails during the harvest season, leave the potatoes in the bed for about 3 weeks. It takes this time for a fungal infection to appear externally in the form of a greyish shell with dents. This gives you the opportunity to sort out infested tubers in good time.

combat

If the damage patterns described become apparent, there is an immediate need for action to save the potato harvest . There is a good chance if you can identify the symptoms on the leaves early and prevent them from spreading to the tubers. The following preparations are currently approved for use in allotment gardens:

• Atempo Copper Fungus Free
• Bayer Garden Vegetables Mushroom Free
• Pilzfrei Cave
• Fungisan vegetable-fungus free
• Vegetable-fungus free polyram
• Vegetable-fungus free Saprol

The preparations are based on different active ingredients that have the common goal of preventing the onset of the disease. The fungal spores are also eliminated in bad weather, because it is precisely then that they multiply within hours. This list is only a snapshot. The database of the Federal Office of Consumer Protection and Food Safety regularly publishes an up-to-date list of all preparations that are approved for the home garden.

Damage to tomatoes

A classic symptom of late blight in its early stages is the vaguely defined, brown-colored spots on stems and leaves. If the damp weather lasts longer, a whitish fungal coating develops on the undersides of the leaves. Now the infection is progressing rapidly, with the foliage turning black and wilting. The shoots soften and break off. Glassy, ​​light brown spots appear on the fruits. The shell bursts open and the tomatoes fall to the ground. Small fruits stick to the plant and turn into mummies. Meanwhile, spore multiplication continues, which increases the risk of infestation.

preventive measures

Hobby gardeners who grow potatoes in their garden always include their tomato plants in the prophylactic procedure. The fungal spores do not overwinter in infected tomatoes, but only in potatoes. If the disease breaks out, it only takes a few hours for them to find their way to the tomato bed. In contrast to potatoes, no really resistant tomato varieties have emerged so far. After 2 weeks at the latest, even the most robust varieties will be afflicted by late blight if it is raging in the garden. Small-fruited tomato varieties still have a chance of surviving until dry weather sets in and the fungi retreat. However, you are not completely defenseless:

• Always plant tomatoes in sunny, airy locations
• A mixed culture with garlic, onions, marigolds or cress is good
• Ideally, breed as a single shoot, use up early and regularly
• If the lower leaves touch the ground, they are removed
• In principle, cultivate under a rain cover (tomato house) or in a greenhouse
• Always disinfect garden tools before, during and after work
• Clean plant debris from support rods repeatedly with hot soda water
• A layer of grass or straw mulch between the rows of plants prevented splashing water

As part of the daily care work, experienced hobby gardeners also influence the defense against late blight. They always water their tomatoes in the early hours of the morning and never over the leaves and flowers. If it is necessary to spray the leaves and shoots with a preparation, this is also done immediately after sunrise. The moisture then has enough time to dry up. After a downpour, the tips of the leaves sometimes bend upwards. Then cut them off without further ado so that the water drains off better. The following biological tonics have proven themselves:

• horsetail broth
• Garlic Peel Tea
• Moose extract, Liver mousse extract
• Rhubarb Leaf Tea
• Onion Skin Tea

Whichever natural tonic you choose; always add a little valerian flower solution and always apply it very thinly to the tomato plant.

Ausgeizen is one of the indispensable care work on tomatoes. As this causes injuries to the tissue, the risk of infection by the fungal spores of Phytophthora infestans increases. Prudent hobby gardeners then treat the plant with an onion skin extract. To do this, pour 1 liter of water over 20 grams of onion skins and leave the mixture to stand for a few days. The result is a thin, brown tincture with which you brush the tomato plant. The treatment not only inhibits the fungal spores, but also keeps pests away.

Incidentally, a plant protection warning service is available from the Chamber of Agriculture in every federal state. Here you will find current warnings regarding the risk of infection from late blight.

combat

If it hits your garden, there’s no reason to panic right away. As an immediate measure, remove all visibly infested parts of the plant and dispose of them in the household waste. This also applies to fruits that have fallen on the ground. If you don’t get used to the use of industrial fungicides, especially for tomato crops, a new product is making waves among organic gardeners. As it is still in the assessment phase, the database for crop protection products does not yet list the preparation. However, it may already be purchased and used: Myco-Sin from Biofa. It is an agent based on sulfuric acid clay, which strengthens already weakly infested plants to such an extent that the fungal attack can be repelled. Myco-Sin can be mixed with other natural agents such as moss extract or horsetail tea to strengthen it. There is still a lack of well-founded field reports.

Alternatively, choose from approved fungicides to prevent further spread. The trade currently offers the following preparations:
Compo Ortiva Spezial Pilzfrei

• Cave Tomaten Pilzfrei
• Detia plant-fungus free
• Infinity

Please note that these agents are of chemical origin and are therefore by no means completely harmless to people and the environment. Strictly follow the manufacturer’s instructions when applying.