End-of-Cycle Diseases (CFDs) begin their injuries long before the end of the cycle. These pathogens are externalized in the final part of the cycle, but they are present many times before, requiring early control, as they consequently reduce production potential and some of them can cause losses of up to 100% (EMBRAPA, 2010). Climatic conditions are factors that must be observed as they can favor or even disadvantage these pathogens. For phytopathogenic diseases in general, proactivity and prevention are of paramount importance, since after establishment their control becomes more complicated. In addition to compromising productivity, these pathogens can cause problems in successor crops.
The spread of one culture to another may vary according to its characteristics. Biotrophs need a living host for their survival and are normally found in weeds or so-called raccoon plants. Necrotrophic fungi have the ability to survive in cultural remains, considering that their host does not need to be alive.
The following diseases can be cited as examples of DFCs: Anthracnose, Brown Spot, Leaf Blight and Frog's Eye Spot.
Anthracnose
Caused by the fungus Colletotrichum truncatum, under conditions of prolonged leaf wetness and temperatures between 18 and 25°C, it is spread through infected seeds and crop residues. The first symptoms of pathogen inoculation occur in the vegetative stage (V1), with necrosis of the cotyledons and subsequent infections in the stems and leaves. At the reproductive stage, the inoculum is transmitted to the flower, making fertilization and grain formation difficult, and may also cause pod abortion, or transmitted to the legume, contaminating grains in formation and causing pods to open. Straw can favor the survival of the C. truncatum fungus as mycelium, due to its greater moisture retention, favoring the germination of spores that will serve as a source of primary inoculum in the following harvest.
Brown Spot
Also known as Septoriosis, it is a foliar disease caused by the fungus Septoria glycines, which survives in crop remains, is more intense in regions of high humidity and temperatures above 25°C, the spores are easily spread through water and wind. . The first symptoms are small brown spots on the primary leaves, which evolve into dark brown spots with a yellowish halo and centers with angular contours. The disease may intensify at the R5 stage (pod formation), leading to severe defoliation and early maturation. The fungus S. glycines is favored by
inoculum reservoir in the straw, which also provides a favorable environment for its development.
Leaf Blight
Caused by fungi of the genus Cercospora, the best known of which is Cercospora kikuchii, also responsible for the purple spot on soybean seeds, it is introduced via infected seeds and crop residues. Its most severe damage occurs in rainy, high humidity and hot regions, with temperatures between 23 and 27°C. Evidence of the disease appears on leaves, stems, pods and seeds. Symptoms present on the leaves are dark to reddish-brown spots, with irregular edges, which can expand to form dark spots and cause blight and premature defoliation. On stems, the spots are red, limited to the cortex. On the pods, red spots appear that may evolve into reddish spots.
Furthermore, it is through the pod that the fungus reaches the seed and causes the purple stain on the seed coat. In severe conditions, infected seeds are less vigorous. Fungi of the genus Cercospora are greatly influenced by straw, where they survive during the off-season, as moisture retention provides a favorable climate for the production and dissemination of spores, in addition to protecting them from external factors.
Frog's Eye Spot
Leaf disease caused by the fungus Cercospora sojina, is spread through infected seeds, crop residues and spores carried by water and wind, develops in conditions of high humidity, high temperatures and dew formation, with initial damage to flowering between stages R1 and R3. Its symptoms appear on leaves, stems, pods and seeds. Symptoms evolve on young leaves from small soggy spots to rounded spots. On the adaxial side, the center of the spots is light brown and the margin is reddish-brown, while on the abaxial side the spots are grayish due to sporulation of the fungus, which can occur early defoliation. On the stems it presents elongated lesions with a grayish color in the center and reddish-brown edges. On pods, the lesions are circular and dark brown in color.
Seeds, when infected, have brown to gray spots and may be cracked. As already mentioned, fungi of the genus Cercospora remain viable in the straw, thanks to soil moisture and reduced temperature, favoring the onset of infection in the following harvest
Weeds as hosts of DFCs
Some weeds can act as hosts for pathogens that cause DFCs, allowing them to remain outside the soybean crop and also causing risks for subsequent crops, given that when their presence is high, the inoculum pressure in cultivated areas increases. Among the weeds capable of harboring pathogens, the following stand out: Horseweed (Conyza spp.), Caruru (Amaranthus spp.) and Picão-preto (Bidens pilosa) as the most susceptible to serving as hosts for leaf spot fungi.
Horseweed (Conyza spp.)
A species of annual and erect plants, they are propagated through seeds. Its leaf tissue is capable of harboring fungal spores, such as those causing Cercospora kikuchii and Septoria glycines. Therefore, the spores can survive during the off-season and, under ideal conditions, spread to other crops through
sporulation or wind..
