Figure 2-23 Bacterial Wilt of Satsuma Citrus

1. diseased branches and leaves. Longitudinal section of diseased rootstock, scion and joint

Don't call it Wendu disease. The disease mostly occurs before and after fruit picking in autumn and winter and the following spring, and it occurs quickly in winter. At the beginning of the disease, the leaves at the top of the crown were green, lost water and rolled, and then spread to the whole plant at the lower part of the crown. The color of the leaves has not faded, just like drying in the sun. After 3-5 days of onset, the diseased plant quickly withered and died from top to bottom. When you shake the branches. Some leaves are easy to fall off, others are not easy to fall off, thus forming a sparse or bare crown. According to the observation of Daoxian Garden in Hunan Province, it takes about 30 days for the leaves to wither and dry until they fall off completely. When the leaves lose water and droop, the branchlets become soft, dry and die, and then gradually expand into perennial branches, or even the whole plant dies. The diseased plants are affected by the whole plant, but also by one or several large branches.

There are two kinds of spring onset: sudden onset and slow onset. Sudden onset is rapid, and leaves wither, roll up and die 3-5 days after onset, and quickly spread to the whole plant. If not pruned in time, the whole plant will die within a week. The slow type is characterized by delayed germination of spring shoots or a small number of delicate spring and summer shoots, especially leafless flowers. After the flower withers, the petals and filaments do not fall off, and the flowers and fruits are abnormal, hanging on the tree, with serious fruit drop and few fruits.

The diseased trees or branches before fruit picking are often weak in summer and autumn, and the fruit is colored about half a month in advance, so it is not fully mature, the fruit is slightly sour, and the peel shrinks, making it difficult to peel off.

A large number of anthrax and Fusarium often rot on dead branches. The diseased plants gradually died from top to bottom, but the bark was intact, the roots did not die, and the roots grew normally. It is often observed that when the bark of a diseased tree is cut off or cut longitudinally, the color of the rootstock and scion of the diseased tree is different, and the more serious the disease is, the more obvious the difference is. The xylem of the scion is yellow-brown, dry and short of water, with some coarse brown to dark brown twill and pits on it; The xylem of rhizome is usually white and grows normally. Observing the cross section of diseased tree graft under microscope, it was found that some ducts were blocked by yellow-brown glue. Sick trees have no or few new roots.

The cause of disease

The pathogen is still unknown. Bacterial isolation and inoculation test and grafting infection test of diseased skin (bud) were unsuccessful. One view is that it may be caused by incompatibility between rootstock and scion; Another view is that it may be caused by virus or bacteroides infection.

Occurrence law

The disease mainly occurs on satsuma mandarin, which harms the middle and late maturing strains, and the early maturing strains are basically not harmed. The incidence of different rootstocks and panicle combinations is obviously different. Citrus with lime or lime as rootstock is the most serious; Rootstocks such as Fuju, local lemon and sweet orange also have diseases; Citrus with trifoliate orange as rootstock is not easy to get sick. The seedlings of mandarin oranges were not hurt. Nanfeng tangerine and local citrus with lime as rootstock also have diseases.

Seedlings and young trees rarely get sick, and trees aged 6-8 years are the most likely to get sick, while seedlings and grafted seedlings over 25 years are not sick. Some patients began to get sick 9- 10 years after colonization.

The disease can occur all year round, and the peak period is from fruit picking in winter to May of the following year, especially in late spring and early summer. Guangxi and Hunan occur more frequently in April-May. After a long period of cold and rainy weather, the illness is the most serious when the weather turns fine.

Citrus orchards in low-lying areas near water are early and heavy, while citrus orchards in hilly areas are late and light. Citrus orchards with partial application of nitrogen fertilizer and less application of organic fertilizer have more diseases. The nutrition level of fruit trees is high in the early growth stage, and if they lack nutrition after entering the full fruit stage, the diseases will be serious.

Prevention and cure method

1. Seedling selection and rootstock selection

In the breeding of satsuma mandarin, Fructus Aurantii was selected as the rootstock, and the early-maturing (Xingjin, Miyagawa, Songshan and Forest) and extra-early-maturing (li mountain, Zaojin 1) strains of satsuma mandarin were given priority.

2. Cut off the diseased branches again

In the early stage of the disease and orchards with mild illness, pruning should be carried out as soon as possible, 4-5 diseased branches should be cut off, and fertilizer and water management should be strengthened, so as to facilitate the formation of COVID-19 of diseased trees, restore growth and enhance tree vigor.

3. Relocation of anvil replacement or high grafting of seed replacement.

In the early stage of the disease or in orchards with mild illness, replace the rootstock with Fructus Aurantii, which can quickly restore the tree. In seriously ill orchards, all the parts above the grafting interface of diseased trees should be cut off and waxed in early spring. In the future, two or three grafted varieties, such as sweet orange and ponkan, will remain in the rootstock buds that are not harmed by bacterial wilt. So that production can be resumed in 2-3 years.

4. Strengthen water and fertilizer management

Do a good job in the irrigation and drainage system of citrus orchard to avoid water accumulation; Increase the application of organic fertilizer, cultivate soil and plough deeply, promote root growth, strengthen tree potential and improve disease resistance.