Brown spot disease, also known as Alternaria citrus brown spot disease, is a common disease in citrus cultivation. It is also nicknamed "black scar" or "black spot disease" by fruit growers. In Guangdong, people are also Xi to call it acute anthrax. Brown spot disease mainly affects citrus and its hybrids, such as Gong Mandarin, Mogu Mandarin, Ehime 38, Mercot, Red Mandarin, etc. This disease is characterized by an extremely rapid onset and easy recurrence, posing a serious threat to the citrus industry.
Brown spot disease is mainly affected by young shoots, flower buds, leaves and fruits. Leaves are one of the main targets of brown spot disease. Once infected, newly germinated young leaves will appear pinhead-sized black-brown oily spots with relatively small lesions and inconspicuous yellow halos around them. Leaves that have spread their leaves but have not yet hardened are the most susceptible to infection, and once the disease occurs, the black-brown spots will expand rapidly, and the center of the lesion will turn grayish-white, forming a tail whisker-like lesion. Some of the leaves that have hardened may also be infected, the lesions are brown and nearly round, as the disease progresses, the center turns grayish-white, these leaves usually do not fall off, and when wet, a black-brown mold layer is produced on them, which is the conidia of the pathogen and the source of primary and secondary infection in the field.
New shoots are also one of the important sites of brown spot disease. When the shoots are infected, pinhead-like black-brown pitfalls initially appear, then the lesions rapidly expand to form oval or fusiform shapes, and when wet, the lesions continue to enlarge until the upper part becomes black-brown and dies. In severe infection, the lesions can spread to larger shoots, causing the previous year-old shoots to die.
Brown spot disease can also infect flowers. The flower stalk and flower bud will undergo brown and die changes, which affects the flowering period and normal development of flower buds of citrus.
The juvenile and enlarged stages are also dangerous periods for brown spot disease. Young fruits that have just dropped their flowers may be infected, and the lesions are small, pinhead-shaped, black-brown, and indented, and most of the young fruits fall off quickly. At the fruit swelling and ripening stages, the typical lesions are nearly round, brown, with a central depression and a distinct yellow halo around them, and these diseased fruits usually fall off early. In addition, grayish-white cork micro-raised lesions are formed on the fruit, which can be easily peeled off.
Brown spot disease is transmitted mainly through Alternaria bacteria, which overwinters mainly on diseased mature leaves. As the temperature rises, lesions on diseased leaves produce conidia, which are transmitted by air currents to new shoots, young leaves, and young fruits, and when conditions are right, these conidia germinate and produce toxins, leading to the death of contact cells, resulting in necrotic plaques. Germs use these necrotic cells to obtain nutrients, grow rapidly and produce more toxins, which spread along the ducts, accelerating cell necrosis and the expansion of lesions, eventually causing leaves to fall off. The occurrence of brown spot disease is related to rainfall, but dew is also enough to trigger the onset and development of the disease. At a suitable temperature (20~29 degrees Celsius), the leaves are continuously moistened, and the infection of the bacteria can be completed in only 8 hours.
The incidence of brown spot disease shows a certain seasonality. The most severe periods of disease are the spring shoots, young fruit stage and fruit expansion stage. Among the new shoots, the incidence of spring shoots was the most severe, followed by autumn shoots, and the incidence of summer shoots was relatively mild. This pattern makes it necessary for growers to take control measures at these critical times.
The occurrence of brown spot disease is related to a variety of factors. On the one hand, orchards with poor cultivation management and heavy pests and diseases are more vulnerable to brown spot disease. Orchards that are low-lying and prone to water accumulation and orchards that are not well ventilated and light are also more susceptible to disease. In addition, orchards with weak trees, more fruits, partial application of chemical fertilizers, and insufficient soil organic matter content are also susceptible.
In order to effectively control brown spot disease, fruit farmers can adopt a series of comprehensive control measures. First of all, it is very important to choose disease-free seedlings. Seedlings need to be sterilized before introduction, and it is best to use detoxified seedlings. In addition, it is important to improve cultivation practices. When building a new orchard, choose a plot with high terrain and good ventilation and light transmission for planting, and plant reasonably densely to promote ventilation and light transmission. For orchards with high humidity, drainage measures can be taken to reduce the humidity and create conditions that are not conducive to the growth of pathogenic bacteria. Tree management is also crucial, avoiding over-irrigation and over-application of nitrogen fertilizers, and increasing organic fertilizers, phosphorus, potassium fertilizers and medium and trace elements in moderation. In addition, the use of biostimulants helps to enhance the tree, improve the disease resistance of citrus, and promote the neatness and rapid ripening of new shoots. Pruning in winter or early spring to remove diseased branches helps to ventilate and let in light. For orchards with severe shading, large pruning is also necessary to improve the growing environment of the fruit trees.
Chemical control is also an important means of controlling brown spot disease. According to the pattern of onset, spring shoots 3
The 5cm hour and the flowering period are the key control times. During these periods, different types of fungicides can be selected for spraying, including long-acting prophyltics and therapeutic agents. During the young fruiting stage, it is also necessary to closely observe the weather conditions, inspect the orchard, cut off the diseased branches and leaves in time, and destroy them. In the young fruit stage, a combination of therapeutic and protective agents can be selected to improve the control effect. During the summer shoot period, it is necessary to erase or spray in time to control the growth of new shoots. The autumn shoot period is carried out again according to the actual incidence of the disease1
2 sprays. Different types of agents are selected at different stages to improve the control effect.
Effective chemical control agents include protective agents such as copper preparations, zinc propazene, mancozeb, fluamine and chlorothalonil, as well as therapeutic agents such as acetamide, triazole fungicides, pyraclostrobin, pythium, iprodione, prochlorazine, and imazole. When using chemical agents, it is necessary to pay attention to the use of protective fungicides before rain, and each type of protective drug should not be used more than 4 times a year, and it is best to use them alternately. Protective fungicides and therapeutic fungicides should be used reasonably, and regular supplements should be made, especially in the case of uncertain weather. For varieties that are susceptible to brown spot disease, such as mandarin oranges, it is also necessary to pay attention to the control effect of anthracnose when using pesticides.
In summary, brown spot disease is a common but challenging disease in citrus cultivation. In order to effectively control brown spot disease, fruit farmers need to adopt comprehensive control measures, including the selection of disease-free seedlings, improved cultivation practices, enhanced tree management and chemical control. Only in this way can the healthy development of the citrus industry be protected and a good harvest can be ensured.