Cercospora leaf spot

Many species of Cercospora can cause orchid leaf spot disease. The symptoms and control methods of orchid leaf spot disease caused by its main species are described as follows.

1.Cercospora epipaczlddis

(1) harm: the pathogen was found in Italy, Germany, the former Soviet Union and the United States.

E.palustris, Shrimp Ridge Orchid, Fritillaria, Cymbidium, Taro Orchid, short-petal Orchid, Crane Orchid, Magnolia and other varieties.

(2) symptoms: in Hexiang orchid, an outer-sized sunken macula appears on the back of the leaf at the initial stage, and soon the spot can be seen on both sides of the leaf, which expands in just a few days, and finally becomes a sunken purple-black raised edge with slightly darker color. The average diameter of aging plaque is about 2~5cm or slightly larger, some plaques may combine into larger irregular plaques, and the center of older plaques is often broken and shattered. When the plaque expands beyond the thicker leaf veins, the tissue remains broken and backward, and the leaf veins remain intact.

When Bletia purpures is infected by the pathogen, small oval or round gray areas usually appear on the back of the leaves at first, and finally infection can be seen on both sides of the leaves. With the expansion of the disease spot, the center of the disease spot becomes purple-black. When the pathogen has been completely colonized, all leaves often have a large number of diseases.

(3) Control: 50% wettable powder against the above leaf spot disease of orchids and all the genera.

All the leaf spot diseases caused by Cercospora have significant control effect. The dosage can be 1000 times, and add a small amount of wetting agent, once every two weeks, a total of 3 times, and then spray with the same concentration once a month if necessary.

2.Cercospora dendrobii

(1) harm: the pathogen mainly harms Dendrobium orchid varieties, it generally does not cause orchid death, but produces many spots on the leaves, so that the orchid loses its ornamental value.

(2) symptoms: symptoms first appear on the back of the leaf, showing a yellowish area, this disease spot will continue to expand in a round or irregular shape, and finally may cover the entire back of the leaf. Irregular yellowish spots are usually produced on the leaves, the disease spots will sink slightly purple-black when they are mature, and the edges of the expansion will remain yellow. Yellow-green spots will soon appear in the corresponding parts of the front of the leaves, and as the disease spots mature, they will gradually become lavender black or black, showing symptoms similar to mites, and the serious leaves will shorten their life span and finally fall off from the plant.

(3) Prevention and cure method: apply 1000 times solution of 50% wettable powder of benzoate, and add small wetting agent, once every two weeks, a total of 3 times, the effect is excellent, and then use the medicine once a month according to the above dosage to prevent plant from being re-infected. It is also effective to control the disease with thiram, the concentration is 80O~1000 times, and the serious disease area must be sprayed for the second time after 5 days.

3.Cercospora angraeci

The main results are as follows: (1) damage: the disease has been found in the United States, Germany and the United Kingdom, and its hosts are: Angraecum fragrans,Odontog lossum crispum (O. alexandrae), Angraecum eburneum,A.comorense,A.orchidglade,A.veitchii and so on.

(2) symptoms: first, it appears on the back of the leaf, which is the size of a needle, brown or light brown sunken disease spot, and finally the disease spot becomes dark purple-brown. As the disease develops, the disease spot may appear near a circular pattern. In many cases, the whole leaf is covered with disease spots, especially the old leaves closest to the base. The corresponding area on the front of the diseased leaves is yellowish green, usually only the 6 or 8 leaves at the base produce serious disease spots. But if the plant is planted too densely, the old leaves near the top may also be infected.

(3) Prevention and treatment: 10O0 solution of 50% wettable powder was used every two weeks and sprayed with Xiaoxu wetting agent for 3 times, and then reapplied once a month if necessary. Chlorothalonil or thiram could be used instead of benzoate to protect the healthy part. In addition, plant spacing should be maintained to facilitate ventilation, and if possible, all diseased leaves should be removed and destroyed.

4.Cercospora cypripedii

The main results are as follows: (1) damage: the disease occurs in the northern United States, southern Canada and the Philippines. Its hosts are Cypripedium candidum,C.humile (C.acaule) C. parviflorum var.pubescens,C. Pubescens,C. Spectable (C. hirsutum) and C. reginae,Paphiopedilum philippinense,Cypripedium philippinense.

(2) symptoms: long light red to dark brown or black leaf spots usually appear between the leaf veins. The width of the spot is 1~3cm, sometimes up to half the length of the leaf. Spores are produced on both sides of the leaf, but most of them are produced on the adaxial surface of the leaf.

(3) Control methods: at present, there are no feasible control measures for this disease on Cypripedium or Paphiopedilum, but benate can inhibit the infection of Cercospora fungi on orchids.

5.Cercospora odontoglossi Perillieux and Delacrois

(1) harm: the disease was found in the United States, Brazil, Venezuela and other countries. The species that are more susceptible to these two diseases are Cartland, Brassavola,Brassocattleya,Brassolaelia,Brassolaeliocattleya,Broughtonia,Diacrium,Epicattleya,Epidendrum,Epilaeliocattleya,Epitonia,Laelia,Laelonia,Potinara,Rodricidium,Schombocattleya,Schombodiacrium,Schomburgkia,Sophrolaelia,Sophrolaeliocattleya, Sophronitis and so on.

(2) symptoms: after the Cartland orchid seedlings in the seedling basin were infected, a slightly raised dark brown spot appeared on the back of the leaf, and the leaf front showed chlorosis in the corresponding part, and eventually necrosis. the symptoms of this pathogen infecting the seedlings are often confused with the harm of mites, except that the mite spot is sunken, while the bacterial spot is slightly raised. When the mature leaf occurs, the irregular sunken purple-brown spot appears on the back, and the diameter of the spot is generally O.32~2.54cm, often around a brown or brown edge, the spot can combine into a large patch, and the corresponding part of the leaf front shows yellow sunken spot, which turns purple-brown when it is mature.

(3) Control method: 50% benzoate wettable powder has excellent control effect on this pathogen, and can well control the spread of the disease. The drug is used once every two weeks, and the concentration is 1O00-fold solution for 3 times. After that, it can be reduced to once a month as a preventive measure against the disease. There are other fungicides that also help to control the disease, such as thiram, carbendan and chlorothalonil, which are all at a concentration of 800 times and must be used twice a month. The addition of Xiaoxuan agent is beneficial to cover the leaf surface more thoroughly. Since C.odontoglossi and most other pathogens causing orchid leaf spot enter the plant through the stomata on the back of the leaves, it is very important to make the insecticides cover the leaves completely.