THE NUMBER ONE DISEASE OF CASH CROPS
Pests and diseases are something that every farmer all over the world is scared of when it comes to crop management. Some more specific, others less, some harder to get rid of, others, not that much. There is one in specific that is present in most of the cash crops grown worldwide and it’s quite hard to be eliminated: the white mold.
The disease caused by the fungi Sclerotinia sclerotiorum has been reported in numerous regions around the world (especially where there is high humidity and mild temperatures) and can lead to a yield loss up to 70%. The fungus has the ability to infect more than 400 known plant species, such as soybeans, cotton, dry beans, sunflower, canola, pea, alfalfa, smoke, tomato, potato, among others. But it not only infects the crops of interest, it can also infect weeds such as, gold-button, chick weed and sawdust, hindering any type of cultural management.
All the previous crops are available in the SMART Fertilizer Software database and can be selected for the creation of fertilization plans when using the system. Take a look at our software and our recommendations!
TAXONOMY AND ETIOLOGY
This fungus has as its basic characteristic the formation (after meiosis) of sexual spores called ascospores.
This group’s mycelium (white and cotton like) has the ability to attract and fuse their hyphae when the availability of nutrients in the environment is scarce, thus forming the sclerotia. These structures are usually formed inside the plants, but if moisture conditions are high, they can grow on tissue surface. They are responsible for enabling the Sclerotinia sclerotiorum too survive in the soil between one crop and another.
Regarding the longevity of sclerotia in the soil, it depends on many factors, one of them being the soil depth. Some studies show that when buried at about 10 cm, the sclerotium becomes more viable than if kept on the surface of the soil. An average permanence of the structure in the soil is 5 to 10 years, because they are coated with an outer protection formed by melanin, thus being highly resistant to microbial degradation.
Thedevelopmentofthediseaseistriggeredbyaseriesofevents such as survival, dissemination,infection, colonizationandreproduction, anditisacyclicalprocess.
The disease can be initiated in two different forms, by myceliogenic or carpogenic germination. The first one, in which hyphae develops from the survival structure – affecting the root tissue -, is not as common, and occurs in very few crops, such as sunflowers and some legumes. Carpogenic germination is the most frequent, when sclerotia originates an apothecium, which can release to the environment more than 2 million ascospores, that can survive for 12 to 14 days depending on environmental conditions.
The main form of infection occurs in the senescent petals of the plant, because ascospores require some source of exogenous nutrients and a film of water to actually infect it, even though they are able to germinate on any surface of the tissue. When climate conditions are not favorable to the fungus, mycelium can remain in colonized flowers for up to 144 hours, and resume development if conditions improve. Colonization occurs associated with the release of enzymes that degrade the cell wall of host cells. The oxalic acid produced by the fungus during infection alters the functioning of the guard cells, causing the stomata to open and enabling the invasion by hyphae. The deterioration of the cell wall caused by pectinase provides the pathogen with the carbon source to start its growth. Once the plant is infected, mold can attack other organs of the plant and even reach other wholesome plants through direct contact.
PROTECTING YOUR CROP
Sclerotia can be disseminated through seeds, soils, machinery, water and many others. Therefore, if your farm area is currently “White Mold free”, the pathogen can be introduced to the field via internally contaminated seeds (with dormant mycelia) or any sclerotium that might be in contact with the seeds. The same might happen with agricultural machinery with contact to contaminated soils.
Once the cycle is well known, there are some measures that can be taken in order to avoid contamination and consequently yield loss:
- Beware of high-density crop population and high humidity
- Make sure the seeds are healthy, certificated and treated before sowing
- Crop rotation (with species that aren’t Sclerotinia sclerotiorum hosts)
- Grow on grass straw to reduce the fungi development
- Clean all agricultural machinery beforehand
If those cultural measures weren’t taken, or if they were not effective and the crop is already infested, the solution is to control the infestation with fungicides or on a biological front, with the use of Coniothyrium minitans, an antagonist funghi.
It is worth mentioning that, since the white mold is caused by a pathogen with resting structures that secure fungal survival for prolonged periods, it is easier to act in order to prevent the infestation that to control it once it is already present in the field.
Just like the human body, healthy and well-nourished plants have greater resistance to diseases, and in case of contamination, will have its productivity less affected. With the SMART Fertilizer Software, you can get an instant assessment of the nutritional status of your crop and detect any deficiencies in a few clicks!
The symptoms of white mold are very similar in several species and begin at the junction of the petiole with the stem, where flowers and leaves detached are usually retained. The leaves start showing soaked lesions that quickly increase size until it takes over the organ. In the pods and in the main stem, it is possible to observe reddish spots, of soft consistency that, once evolved, are covered by a dense white and cotton like mass of mycelium, which in a few days can turn into sclerotia. Fruits, tubers and tuberous roots are also attacked and caused to rot when developing a white mold with sclerotia on the surface.
If you are interested in learning more about pests and diseases that cause significant damage to popular crops, keep reading the articles posted weekly here on the SMART Fertilizer website!
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