Identification and Control of Several Leaf Diseases of Poplar

Weishuang TONG, Jingwen HUANG, Huashan GAO

School of Chemical and Environmental Engineering, Pingdingshan University, Pingdingshan 467000, China

Abstract This paper introduces the identification characteristics, occurrence regularity and control methods of several leaf diseases of poplar, including poplar angular leaf spot, poplar large spot, poplar leaf blight and poplar rust, in order to provide the basis for the healthy growth of poplar and the technical support for the sustainable development of forestry industry.

Key words Poplar; Poplar angular leaf spot; Poplar large spot; Poplar leaf blight; Poplar rust; Identification and control

1 Introduction

Poplar belongs toPopulus, Salicaceae, and there are about 100 species of poplars and thousands of hybrid strains in the world. China is the distribution center of poplar in the world, whose species accounts for about half of the total in the world. Poplar is widely cultivated because of the advantages of wide adaptability, easy reproduction, fast growth, good material and easy processing. In China’s major "three-north" shelterbelt project and farmland shelterbelt system, poplar trees are mainly planted. However, in recent years, due to the single tree species in afforestation design and neglect of management, poplar diseases occur frequently, which make the trees lose their use value and even die. This paper introduces the identification characteristics, incidence regularity and control methods of several leaf diseases of poplar, including poplar angular leaf spot, poplar large spot, poplar leaf blight and poplar rust, in order to provide a basis for a healthy growth of poplar and the technical support for the sustainable development of forestry industry.

2 Poplar angular leaf spot

2.1 Identification characteristicsPoplar angular leaf spot can harm the leaves and shoots of poplar, but the spots mainly occur on leaves. In the initial incidence stage, needle-shaped brown small spots appear on the adaxial surface of leaves. With the development of the disease, the enlarged spots are irregular or nearly round in shape restricted by veins, dark brown. In severe cases, multiple spots combine to large plaques. In the late stage, the lesions are gray-white, gray-brown to brown in the center, and gray-green villi could be seen in the adaxial surface of leaves, namely conidiophore bundles and conidia of the pathogen. The leaves are abaxially black in the center and yellow-green in the margin. In the late stage of the onset, a large number of leaves fall off in advance, and almost all leaves fall off in severe cases, leaving only few leaves in the tip of the tree.

2.2 Incidence regularity

2.2.1Climatic environment. The severity of poplar angular leaf spot is closely related to rainfall, fog and dew of the year. There are more cases in August and September, and rainy season or year is conducive to the occurrence and spread of the disease.

2.2.2Host resistance. There are differences in disease resistance among different poplar varieties, and the victim species includePopulustomentosa,P.adenopoda,P.tremula,P.yunnanensis,P.simonii,P.dakanensis,P.alba,P.maximowiczii,P.nigra,P.nigravar. italica,etc.

2.2.3Cultivation management. The standing forests, which are long-term barren, overgrown with weeds, with poor ventilation and high humidity, are conducive to the occurrence and spread of the disease. The standing forests on shady slopes and mountain ridges are more susceptible to the disease. Too dense seedlings and poor ventilation and light will lead to severe incidence. The forest will suffer from severe disease in the next year if it has been severely attacked over the years and has more overwintering bacteria sources.

2.3 Control methodsThe forest should be constructed scientifically to create a growth environment not suitable for the occurrence of the disease. According to different cultivation purposes, the density of afforestation should be reasonably determined, and over-dense planting should be avoided, so as to maintain ventilation and light transmittance in the forest. Scientific fertilizer and water management should be performed, and organic fertilizer must be supplemented, so as to enhance plant disease resistance. Water should be drained timely after rain, to prevent moisture retention. Reasonable pruning is conducted, and sick leaves should be timely removed during the growth period. The fallen leaves in the forest must be cleaned after autumn, which are deeply buried or burned, to eliminate pathogens. Chemical agents should be applied in time in the early stage of the disease to achieve the purpose of comprehensive prevention and control. Before the onset of the disease, 0.5%-1.0% Bordeaux mixture can be sprayed. In the initial incidence stage, 70% mancozeb WP 500-600 times dilution, or 65% mancozeb WP 500-600 times dilution, or 75% chlorothalonil WP 600 times dilution can be sprayed in time, and continuous prevention and treatment is carried out according to the situation.

3 Poplar large spot

3.1 Identification characteristicsPoplar large spot mainly harms leaves, and almost all leaves in the lower part of the severely infected seedlings are scorched and caducous, affecting the normal growth of seedlings. The disease mostly occurs at the leaf margin and around the wound. In the initial stage of the disease, the spot is brown and gradually expands into a large round spot inserted with yellowish-brown and gray-white striations. The spot is large, up to 10 cm, and one spot can account for 1/3-1/4 of the leaf surface. In the late stage, gray-white protrusions are produced on spots, namely acervulus, and spots are connected, resulting in dried-up leaves.

3.2 Incidence regularityIn the years with much rainfall in spring, the onset is early and heavy. The leaves in the lower part of plants first develop the disease, and then the disease gradually spread upward. Trees in which defoliators occur heavily will have heavy incidence. The disease mainly harms seedling, and too dense seedlings and poor ventilation will be conducive to the incidence. In addition, there are differences in disease resistance among different poplar varieties.P.×beijingensisandP.alba×P.tomentosa-2 are the most infected, whileP.albavar.pyramidalisis moderately infected, andP.nigravar.thevestinais mildly infected.

3.3 Control methodsPrevention is the major method against poplar large spot. The growth of trees and their disease resistance can be improved by strengthening cultivation management, improving soil and adding organic fertilizer. Diking and reasonable pruning are conducted, and sick leaves should be timely removed during the growth period. The fallen leaves in winter must be cleaned, which are deeply buried or burned, to eliminate overwintering pathogens. Chemical agents should be applied in time in the early stage of the disease to achieve the purpose of comprehensive prevention and control. 1∶1∶125 Bordeaux mixture, 65% zineb WP 250-500 times dilution, or 50% thiram WP 500 times dilution, or 70% mancozeb WP 500-600 times dilution can be sprayed once every 10-15 d, and continuous prevention and treatment is carried out according to the situation.

4 Poplar leaf blight

4.1 Identification characteristicsPoplar leaf blight mainly occurs on poplar leaves, and also damages shoots and young stems, causing a large number of defoliation in advance. The disease especially infects seedlings and young trees. In the early stage of the disease, there are small faint brown spots on leaves, which gradually expand with the development of the disease. The affected tissue turns yellow, then the central part turns brown, and dark brown molds grow on it, which are distributed on both sides of the affected leaf.

4.2 Incidence regularity

4.2.1Climatic environment. Poplar leaf blight is a kind of disease that likes high temperature and humidity, which generally occurs severely in rainy summer. Conidia need appropriate temperature, relative humidity and acidity when germinating. Generally, conidia can germinate and produce germ tubes under the conditions of 7-38 ℃, relative humidity 93% and pH 2-8, but the germination is the best only at 26-28 ℃ and pH 6. Conidia formed under unstable temperature conditions have a germination rate of less than 30%.

4.2.2Host resistance. Different species and strains of poplar have different disease resistance. It is found that the hose range of the pathogen of poplar leaf blight is wide, and many species in section Tacamahaca, section Aigeiros and their hybrids, as well as section Populus can be infected by the pathogen, especially section Tacamahaca and hybrid of section Tacamahaca and section Aigeiros.

4.2.3Cultivation management. The barren, compact soil and low-lying land easily leads to the occurrence of the disease. In addition, root rot or weak tree vigor caused by water accumulation in the roots can also lead to early and severe incidence. Application of basal fertilizer will lead to lighter incidence than topdressing, and winter fertilization leads to lower incidence than spring fertilization.

4.3 Control methodsPrevention is the major method against poplar leaf blight. Resistant varieties should be selected for afforestation to reduce the widespread spread of the disease. Reasonable dense planting and reasonable pruning are implemented, in order to improve ventilation and light transmission conditions. Reasonable fertilization and watering, and increased application of organic fertilizer and nitrogen fertilizer will enhance tree potential and improve disease resistance. The fallen leaves and diseased shoots are removed in winter, which are deeply buried or burned, to eliminate the source of primary infection. Chemical agents should be applied in time in the early stage of the disease to achieve the purpose of comprehensive prevention and control. 75% Chlorothalonil WP 800 times dilution, 50% procymidone WP 1 000 times dilution, 25% propiconazole EC 1 000 times dilution, 70% thiophanate-methyl WP 800 times dilution can be sprayed once every 7-10 d, and continuous prevention and treatment is carried out according to the situation. When spraying, Xiaoyehuangyejing 800 times dilution or K-Zn-Ca amino acid 1 000 times dilution can be added, which can prevent and control diseases and increase nutrition, enhance disease resistance and stress resistance, and reduce the symptoms of yellow leaf and leaflet caused by nutritional deficiency.

5 Poplar rust

5.1 Identification characteristicsPoplar rust mainly affects seedlings and young trees, and also occasionally occurs in large tree. The disease harms leaves and buds and affects tree growth. In the spring, when poplar leaves are spreading, yellow powder pile can be seen on the tree, which looks like a bunch of malformed buds of yellow hydrangea, and the infected buds die soon. Small yellow spots are formed when leaves expanded from normal buds are infected, and scattered yellow powder piles can be seen on the abaxial surface of leaves, namely uredinium. In severe cases, the uredinium can unite into a large area, and the abaxial surface of the affected leaves is raised. The infected leaves will fall off ahead of time. In colder areas, a few reddish brown, nearly round or polygonal pocks can be seen on diseased leaves in early spring, namely teleutosorus of the pathogen.

5.2 Occurrence regularityThe pathogen survives the winter in winter buds or as uredospors on diseased leaves. In the following spring, with the increase of temperature, the winter buds begin to activate, and the overwintering mycelia gradually develop and form uredinium. The infected buds could not expand normally, and form malformed buds fully covered with uredinium. These buds become the center of primary infection. Although some uredospores on the diseased leaves have the ability of germination and infection, their germination ability will rapidly lose with the gradual increase of spring temperature. Therefore, infected winter buds are the main source of primary infection.

The lowest temperature for uredospore germination is 7 ℃, the highest temperature is 30 ℃, and the optimum temperature is 15-20 ℃. The incubation period of the disease is also closely related to temperature. When the average daily temperatures are 12.9 ℃, 15.2-17.1 ℃ and 20.3 ℃, the incubation periods are 18, 13 and 7 d, respectively. In the optimum temperature, the higher the temperature, the shorter the incubation period. Humidity is another key factor for the occurrence of the disease. When the relative humidity in the forest is above 85%, it is conducive to the occurrence and development of the disease. In dry years and areas with little rain, the disease occurs mildly.

5.3 Control methodsThe forest management should be strengthened, and diseased buds and fallen leaves in the field should be removed timely. Diseased buds are easily found because of its bright color, and the number is generally not more than 0.3%. The picked buds should be put into plastic bags to prevent uredospores from spreading, and destroyed in time. During the onset period, 25% triadimefon WP 800 times dilution can be sprayed. When the leaf buds are opened, the agent is sprayed once before dispersion of uredospores, which has good control effect on the disease. Most uredospores of this pathogen land within 300 m from where they are produced. Therefore, seedbeds should be set up more than 300 m away from severely infected nurseries to reduce infection.