ANTHRACNOSE, THE DISEASE IN CROPS
Today we have to talk about something as important as plant health. As we have seen, there are many ways to protect our crops, starting with prevention and ending with the application of chemicals or pesticides. There are many solutions for the same problem (not all of the same quality), that is why we want to see this disease from all points of view. Go ahead!
WHAT IS ANTHRACNOSE?
Anthracnose is a disease typical of hot and humid climates, and it is classified among several genera of fungi, such as Gloeosporium or Colletotrichum.
Although it affects many crops, the diagnosis is usually similar, since small spots appear on the cuticola (both leaves and fruits) that end up increasing in size and necrotizing over time.
It is a fairly frequent fungus in vegetables, where planting frames are quite small (to increase the number of plants per surface area), and aeration is reduced. Under these conditions, the disease develops relatively easily, provided there are adequate humidity and temperature conditions.
Anthracnose is a well-known disease for potato and tomato growers, although it will always depend on the area where they grow. It is also present in crops such as carrots ( Alternaria dauci ), as well as in crucifers ( Alternaria alternata ).
If the plant has suffered injuries, either by mechanical blows (tractor pass, friction, insect attacks), or suffers some nutritional imbalance, the chances of the crop becoming infected are increased, since these fungi take advantage of the wounds to enter and be divided.
Although there are different types of anthracnose, a very common one that usually affects mandarins (Fortune, Nova, Murcott, Minneola, etc.) preferentially is Alternaria alternata . It affects both fruits, leaves and shoots, causing circular spots of brown color that expand following the line of the leaf veins. If the leaf is very affected, it falls to the ground, so a very common symptom of the disease is to see an important defoliation in the citrus.
anthracnose in mandarin, Source: IVIA
The anthracnose corn is caused by the fungus Colletotrichum graminicola. Symptoms are characterized by necrotic lesions with yellow halos and various shapes. These lesions can exceed 1.5 cm in length and form chlorotic rings at the edge of the stain.
Be careful with the seeds and fruits, as they can be carriers of the disease.
Anthracnose is a well-known disease in olive cultivation . It is caused by the fungus Colletotrichum acutatum and Colletotrichum gloeosporioides and is commonly known as the soapy olive . This disease especially stands out in humid areas dedicated to olive growing.
In autumn-winter there is a rot in the fruits where necrotic spots appear, with depressions and circular in shape. Little by little the affected area turns a brown color that little by little it spreads. The olive becomes dehydrated and the oil loses great quality.
It appears when: the relative humidity is very high (85-95%) and the temperatures are mild-high (from 10 ºC to 30 ºC).
SOLANACEAE AND OTHER HORTICULTURAL
Anthracnose is also a typical disease in vegetables such as tomatoes or peppers, especially in high humidity conditions. It produces circular spots of brown or ocher color on both green and ripe fruits. With adequate relative humidity and high temperatures (such as in greenhouses), anthracnose is likely to spread rapidly.
Tomato affected by anthracnose
The infectious agent is Colletotrichum Graminícola , and it affects a large number of cespitose species. This is the case of Ray-Grass, Poas, Festucas and Agrostis, among others. It requires humid weather and high temperatures (above 25 ºC), and the disease worsens when soil conditions are not the most suitable. Hence the importance of avoiding compacted soils in lawn cultivation.
The disease is observed and identifed when stands of yellowish turf are seen with black and dark patches. These stains can spread easily if the weather is humid and the lawn stays wet most of the day.
There are many more affected crops:
- Carrot ( Alternaria dauci ) , known as carrot leaf blight.
- Olivo (Colletotrichum acutatum y C. gloeosporioides).
- Stone and pip fruit trees.
- Banana trees and poplars.
- Ornamental plants (shrubs, climbers, trees, etc.).
HOW IS DISEASE PROMOTED?
A technique that the farmer usually does and that inevitably spreads anthracnose, is to leave crop residues in the soil, in order to be able to give up nutrients that, in the future, the plant will absorb.
Anthrancosis in cranberry. Source: agf.gov.bc.ca
Although it is an efficient technique and that means saving costs for the farmer, it must be abandoned if the previous crop has shown signs of suffering from anthracnose. This is because this disease is able to overwinter in said remains that remain in the ground, waiting for new plants to be grown to move and infect them.
When the plant is not very vigorous and is growing, the infection occurs in the lower third of the plant, entering through the epidermis.
The environmental conditions are those that we have commented previously. Times of humidity (after a series of rainy days where dew appears), and with warm temperatures that exceed 15 ºC (in the case of anthracnose in potatoes ) or above 25 ºC in other crops (for example, in grass).
Regarding humidity , although we have mentioned that it must be high, the frequent values of the appearance of the disease are higher than 90%, in most cases. If the rainy intervals alternate with moments of wind, the propagation and dissemination of anthracnose spores is much greater.
Beware of planting affected seeds! The mycelium of anthracnose is able to survive in the seed, and even among the cotyledons of a seedling. Hence the importance of treating the seed with appropriate fungicides, if there is a risk that the disease may be present.
In horticultural crops, anthracnose usually appears on the lower leaves of the crop, since, logically, they are the longest on the plant and are more susceptible to being “visited” by the fungus. In the initial stages of the disease, small circular spots appear, which turn dark over time. This process is known as necrosis and it spreads throughout the leaf.
A peculiarity of the affected tissues is that concentric rings are formed, where the dead tissue is sunken. In some cases, such as the initial photograph of anthracnose in pepper , you can see how there are a series of concentric rings, where the central one is darker than the rest, and the outer edge has a dark brown coloration or chlorotic ring.
In fact, a relatively simple way to distinguish anthracnose from other diseases is to see this chlorotic ring (initially), which progressively turns dark.
In the leaf, curiously, the fungus does not cross the veins of the leaf, so it can often be seen how the progression of the disease stops when the anthracnose encounters the “barrier” of a nerve of considerable size. This particularity is also an ideal way to identify and distinguish this disease.
Crop affected by anthracnose. Source: fieldcropnews
METHODS TO PREVENT AND CURE ANTHRACNOSE
A well-known anti-anthracnose instrument has been copper. Different concentrations of copper oxychloride, cuprous oxide, cuprocalcium sulfate or tribasic copper sulfate can be found on the market.
There are also different active ingredients that are effective against this disease. To find out what they are, we put this link from the Ministry of Agriculture, and thus we do not mention assets that may disappear in the future. As you know, the phytosanitary market is currently moving at breakneck speeds.
As the chemical fight is a privilege to cure this disease that ecological agriculture cannot allow, cultural work is of great importance. The farmer must be prepared to face and reduce (above all, prevent) the effects of anthracnose when temperatures are high and days of humidity are expected. These factors are conducive to the development of the disease. Given this, the farmer has several courses of action:
- Space out the plantation frames when a climate conducive to this disease is expected.
- Use ridges and other instruments that improve the aeration of the crop.
- Improve soil drainage and avoid irrigation water puddling.
- Avoid leaving crop remains in the soil affected by the disease, as it is a means of propagation, whose mycelia can remain active for 2 or more years.
- Practice crop rotation, avoiding monoculture and increasing the pathogenic density in the soil (nematodes, fungi and soil insects).
- Plant anthracnose resistant or tolerant varieties.
Regards and until next time! 🙂