Plant leaves are an important reporter of diseases and various unhealthy conditions of plant species. As such, they deserve to become the object of constant attention by the farmer.
But what are the main detecting signs? And how can we recognize plant diseases from leaves?
In this guide we have tried to summarize the main tips to follow and actions to activate in case there are symptoms of potential diseases: let’s find out them together.
Observe leaf spots and discolorations
The first tip we want to focus on is also the most common and obvious one: the presence of leaf spots and discoloration can in fact often be the first sign of a disease.
Not everyone knows, however, that the different coloration of spots and discolorations can give us valuable insights into the health of the plant.
Brown spots
Brown spots on leaves, if they have well-defined edges and a lighter center, can be the calling card ofanthracnose, a fungal disease caused by several pathogens. It is unfortunately a rather persistent disease that can result in very serious damage to the plant. To deal with it, it is necessary to quickly remove the affected leaves and to disinfect the tools used at each cut in order to preserve the healthy parts. One can then use copper- or sulfur-based products or use triazoles and propiconazole.
If the brown spots have the characteristic concentric rings, it could instead be alternariosis, a disease caused by the fungus Alternaria solani: it damages leaves with angular lesions that eventually crumple them up and cause their death, infecting neighboring leaves. Copper-based fungicides mixed with Azoxistrobin and Difeconazole can be used to deal with the condition.
Finally, if the brown spots are circular in shape, with a dark edge and gray center, it could be cercosporiosis, a condition that affects all cucurbits but, with particular intensity, watermelon, cantaloupe melon, and cucumber. Determined by cercospora, a genus of ascomycete fungus, it is coped with by the use of fungicides and-indirectly-by cultural practices such as rotations and removal of infected vegetation residues, followed by deep plowing and seed disinfection.
Black spots
In the case of black spots on leaves, the main suspect is fumigation, a disease that creates a powdery black layer on the surface of leaves. Caused by the development of saprophytic fungi on carbohydrate substrates on the surface of plants, to eliminate it it is necessary to remove unwanted hosts with specific products. In the case of mealybug, it is advisable to use linseed oil and soft soap to dissolve the honeydew and get rid of the fungi. If, on the other hand, aphids should be the cause, soft soap is generally sufficient.
Another disease reported by black spots is scab, a condition caused by Diplocarpon or Marssonina fungi in the case of ornamental plants, or Venturia for fruit plants. Its presence is indicated by velvety black spots that can become crusty. To cope with this condition, it is a good idea to proceed with regular pruning of the leaves to allow air and light to filter in, cleaning the plant of fallen leaves. It is important to take prompt action, removing the affected parts and using specific antifungal products.
Another disease condition reported by black spots is septoria, which generates small, circular dark spots surrounded by a yellow halo. Provoked by Septoria, a genus of ascomycete fungi in the family Mycospherellaceae, it is treated with the use of specific foliar fungicides.
Yellow spots
Yellow spots can also be an indicator of frequent diseases. These include ferric chlorosis, which causes generalized yellowing of the leaves (the veins remain green). Treatment of this condition can be done through the application of chemical acidifiers such as iron sulfate or ammonium sulfate. Organic organic fertilizers composed of sulfur, iron, leonardite and a good content of organic matter are also found on the market. We also mention downy mild ew, a condition that starts with yellow spots that later turn brown, often accompanied by a whitish mold on the underside. Fungicides such as mandipropamid, cymoxanil and fosetyl aluminum are recommended to combat it, with copper-based products such as copper oxychloride and copper hydroxide.
Finally, yellow spots could be an indicator of mosaic virus, so called because it produces a characteristic mosaic pattern of yellow and green areas on the leaves. Targeted interventions such as boric acid, or the use of copper-based products, can be positively evaluated.
White spots
In the case of white spots, in most cases it could be powdery mildew (white sickness), which forms a powdery white coating on the surface of leaves. It is usually combated with sulfur or sodium hydrogen carbonate fungicides, a substance also useful against mildew and scab. We also mention in this category aleurodids, which leave whitish spots due to their secretions.Whiteflies can be countered by cleaning plants with citronella vegetable soap.
White spots could also be a symptom of gray mold(Botrytis), a condition that begins with whitish spots that turn gray and rot as days pass. Recommended treatments are those with copper, sulfur, baking soda, horsetail extract, propolis extract, and thyme or cinnamon essential oil.
Red and purplish spots
Finally, it is also possible for the leaves to be affected by red and purplish spots, which could be a sign of phosphorus deficiency, which can cause such discolorations, especially on the lower page. It may also be anthocyanosis, which causes redness due to heat or water stress, or even bacterial blight: some forms may manifest as reddish or burgundy spots.
Keep in mind, however, that the same disease can show different coloration at different stages of development and that environmental conditions (humidity, temperature, sun exposure) can influence the appearance of the spots. Therefore, for an accurate diagnosis, other factors such as the shape of the spots, their distribution on the plant, and speed of spread must also be considered.
Check the texture and shape of the leaves
Based on what we have premised above, a second piece of advice we can give our readers is to check the texture and shape of the leaves.
In fact, a healthy leaf should be turgid and maintain its natural shape. If, on the other hand, the leaves appear crumpled, blistered, or deformed, then it could be because of an insect infestation or viral disease.
If then the leaves become brittle and dry at the edges, it could also be a problem with irrigation or environmental stress. Finally, if the leaves become soft and watery, it could be an indication of an ongoing bacterial infection.
But how to respond?
We specify that, like all treatments, remedies against soft or dry leaves should be weighted based on the specific condition and type of plant that is affected by this problem.
For example, a soft leaf could be the result of too frequent watering, which causes water to accumulate in the leaves, making them precisely soft in texture. To prevent this phenomenon, or take corrective action, it is best to water the plant only when the plant needs it, following the directions for the specific plant type.
If, on the other hand, the leaves are dry, the problem could be the opposite, with a dehydration situation that can be corrected by watering the soil more frequently or using appropriate soil supplements.
Monitor the progression of symptoms
To take proper care of your plant it is very important to observe how symptoms develop over time and on which parts of the plant they first appear.
For example, if the problem starts on the older leaves and then moves to the upper part, it could be a nutritional deficiency such as nitrogen deficiency. If, on the other hand, symptoms first appear on the younger leaves, it could indicate a deficiency of other nutrients such as calcium.
It is also useful to monitor the speed at which symptoms spread, because it helps to distinguish between environmental problems (which tend to affect the whole plant at once) and infectious diseases (which spread gradually).
For expository convenience, we have created disclosure tables for practical reference.
Let us begin with a look at the information that can be obtained from observing disease progression by location.
| From the margins toward the center of the leaf | It may be potassium deficiency, which results in yellowing and necrosis starting from the edges of the leaf, or excess salinity, which results in scorch starting from the edges of the leaf. Phytophthora also tends to generate rot that advances from the edges and then proceeds to the central part of the leaf. |
| From the bottom to the top of the plant | There are also different causes of worsening disease from the bottom to the top of the plant. Indeed, in this case it may be a nitrogen deficiency, which promotes yellowing of the leaves starting from the older ones, or a magnesium deficiency, which starts from the basal leaves. Again, it could be an indication of fusarium, which causes precisely the yellowing and wilting that progresses from the lower part. |
| From the top to the bottom of the plant | Different may also be the causes if the disease spreads from the top to the bottom of the plant. Indeed, it may be a condition of calcium deficiency, which results in deformation of the young apical leaves as a priority, as well as boron deficiency, which promotes the death of the upper vegetation. Finally, it may be virosis, with mosaicism often starting with the youngest leaves. |
Instead, in this table we try to understand what information we can obtain about the health condition of the plant based on the rate of disease progression.
| Quick (24-48 hours) | A rapid acceleration of disease symptoms is typical of frost or heat damage. In the former case there tends to be immediate blackening of tissues, while in the latter case there will be sudden wilting. It is also possible that such rapid deterioration may be brought about by pythium, with root rot causing rapid plant collapse. |
| Moderate (1-2 weeks) | Deterioration at moderate rates of the plant can be brought about by downy mildew, powdery mildew or alternariosis. As we have pointed out in the past paragraphs, the symptoms are respectively the spread of spots in wet conditions, expansion of white mold, and progression of necrotic lesions on the leaves. |
| Slow (more than 3 weeks) | Such slow disease progression may be driven by severe nutritional deficiencies, virosis, with progressive mosaic manifestation, or bacteriosis. |
Finally, some useful information can also be obtained byanalysis of diffusion patterns and patterns, which show us how the disease and its visible symptoms spread on the plant.
| Uniform | If the disease spreads uniformly throughout the plant, it could be caused by environmental stresses (temperature, humidity), root problems, or established nutritional deficiencies. |
| Spotted | The symptoms of spot disease are usually the result of fungal disease, insect attack, or localized bacterial infection. |
| Sectoral | Deterioration of sector plants is often the result of vascular problems, partial root rot, or localized mechanical damage. |
