Key Points of Prevention and Control Technology against Pear Powdery Mildew

Lijuan Gao,Jintao Xu,Longfei Li,Minghui Ji,Hai’e Zhang,Baofeng Hao

Changli Institute of Pomology,Hebei Academy of Agriculture and Forestry Sciences,Changli 066600,China

Abstract Due to the lack of attention to the prevention and control of powdery mildew in pear orchard in autumn,the occurrence of pear powdery mildew is aggravating year by year,and it has become one of the main diseases of pear tree.In this paper,the symptom diagnosis,incidence regularity and prevention and control technology of pear powdery mildew are summarized,which will provide the reference for prevention and treatment of pear powdery mildew in production.

Keywords Pear powdery mildew;Symptom;Incidence regularity;Prevention and control

Pear is the second largest tree species in north China,and its yield and area account for more than 60% of the world,occupying an important position in the world pear industry[1].However,the occurrence of pests and diseases in the production process will seriously affect the quality of fruits,causing huge economic losses.Pear powdery mildew,Phyllactinia pyri(Cast)Homma,mainly harms leaves,which often occurs in pear producing areas in Hebei,Liaoning,Shanxi and Shaanxi provinces,causing early defoliation in severe cases[2-3].The occurrence of pear powdery mildew has not been paid much attention compared with pear scab and pear black spot.As environmental conditions change,the harms of the disease also change.In eastern Hebei Province,pear powdery mildew rarely occurs in spring,and enters the peak incidence period in autumn.In production,the prevention and control of pear diseases mainly concentrate on the period before fruit harvest.After fruit harvest in autumn,the prevention and control of pear powdery mildew in pear orchard has not been paid enough attention.Moreover,high temperature and drought often occur in recent years,so the occurrence of pear powdery mildew is aggravating year by year.Early defoliation of a large number of pear trees often result in secondary germination during the growth period of pear trees,which has a great impact on the yield and tree potential,becoming one of the main diseases of pear trees[4].The occurrence of the disease is closely related to climate factors.If a prediction model can be established according to the local actual situation,early prevention can be done to reduce losses[5].

1 Diagnosis of Symptoms

Pear powdery mildew mainly harms leaves,which mainly harms mature leaves in autumn.In eastern Hebei Province,the disease generally occurs in July,and enters the peak incidence period in autumn.The disease first appears in the middle and lower leaves of branches,and then gradually spread towards apical new leaves.

When pear powdery mildew occurs,there are nearly round brown spots of different sizes and different numbers on the back of leaves at the base of pear branches,which usually extend to the whole leaf.A pale pink layer usually appears on the spots,which are the conidiophore and conidia of the pathogen.In the late incidence stage,there will be small grains on the spot,which are cleistothecia of the pathogen.The cleistothecia are initially yellow,and distributed sparsely and evenly.The small yellow grains gradually grow up and become brown,and cleistothecia finally turn black brown.The cleistothecia will fall into the soil or pear tree branches,and some remains in leaves.When the disease occurs severely,it can also harm the tender tip,covering white powder on the surface of the diseased tip.

2 Incidence Regularity

P.pyri (Cast)Homma belongs to Erysiphales,Ascomycetes.The external hyphae of P.pyri(Cast)Homma grow and develop on the surface of the diseased leaves.There are three species of pathogens causing pear powdery mildew:P.pyri(Cast)Homma,Podosphaera clandestine and Podosphaera leucotricha, among which P.pyri(Cast)Homma is the most common and the most harmful[6].

P.pyri(Cast)Homma is an ectophyte,which survives the winter as cleistothecia in diseased leaves.When the conditions are favorable in the next year,the cleistothecia will rupture,releasing ascospores.Ascospores will spread with wind and rain and fall on the leaves of pear trees,starting primary infection.The germ tube invades the mesophyll cell from stoma,forming haustorium to absorb nutrients for development and reproduction.After the pathogen infects pear leaves,the hyphae prostrate on the leaf surface,and form conidiophores on the leaf surface,then produce a large number of conidia for secondary infection,making the disease spread continuously.Conidia are white and powdery in appearance and are easily blown away by the wind,so it is a typical aeroborne disease.The conidia stage occupies most of the life cycle of the pathogen,and secondary infection is easily occurred under suitable climatic conditions,resulting in the prevalence of the disease.

The occurrence period of pear powdery mildew is related to region and year,and the incidence period is not consistent among different pear varieties.When there are too concentrated rainfall and continuous rainy for a long time,the orchard humidity is too large,which will easily cause the occurrence of pear powdery mildew[7].The pear orchard with dense planting and closed canopy easily suffer the disease,while those with poor drainage and heavy application of nitrogen fertilizer suffer the severest damage.Pear powdery mildew is harmful to many pear varieties in China,and causes severe damage to Laiyang pear,Qiubai pear,Kangde pear and Huagai pear,while other varieties are less affected[8].

3 Control Technique

3.1 Selection of disease-resistant varieties Using disease-resistant varieties is the most economical and effective way to prevent diseases.In endemic areas of pear powdery mildew,disease-resistant varieties should be selected,while susceptible varieties should be avoided.In eastern Hebei Province,the suitable cultivars are Huangguan pear,honey pear,Jingbai pear,Zhongli 1,Zaosu,etc.,and Qiubai pear,Huagai pear and Bo pear are susceptible cultivars.

3.2 Remove the source of the disease In the orchards attacked by pear powdery mildew,orchard cleaning must be paid attention;leaves must be cleaned thoroughly and burned concentratedly,and soil must be ploughed,to eliminate the primary source of infection.When pruning in winter,diseased branches,diseased buds and diseased shoots must be cut off,burned or buried intensively,so as to remove the overwintering bacteria sources.

3.3 Strengthen cultivation management Cultivation management should be improved,and chemical fertilizer must be scientifically used,such as reducing the amount of nitrogen fertilizer and increasing the application of high-quality organic fertilizer.Nitrogen fertilizer or compound fertilizer is applied by shallow ditch covering method.The fertilizer should not be broadcasted on soil surface to avoid failure or loss.Before pear tree germination in spring or after fruit harvest in autumn,circular furrows are dug at the outer projection of pear tree crown,or radial ditch is dug with the trunk as the center,and mature organic fertilizer is applied at the dose of 1-2 kg fertilizer per 1 kg fruits.After fertilization,the soil is covered,and irrigated.According to the specific situation,when there is a wide space between the rows of pear orchard,green manure can be interplanted in the orchard,or covered with grass in the tree.Increasing the application of high quality organic fertilizer can improve the soil aggregate structure,promote robust growth of root system and tree body,and enhance disease resistance of the tree.

Careful trimming will maintain good ventilation and light in the canopy.For the pear orchard with too dense planting,when extended branches cross with each other,appropriate thinning should be conducted after sheltering.The large branches should be removed year by year if the number of branches left in the crown is too much,and the fruiting branches and small and medium-sized branches that grow too dense should be thinned appropriately.The trees should be reasonably loaded,and excessive growth of trees must be controlled,while the alternate bearing must be balanced.The low-lying pear orchard prone to water accumulation,or pear orchards with dense planting and closed canopy should be timely ditched,and reasonably shaped and pruned,to improve the orchard ventilation and light conditions,which can effectively reduce the incidence of pear powdery mildew.

3.4 Chemical control In spring,the pear tree is sprayed with lime sulphur(1-3 Baume degree)before germination to kill the overwintering bacteria and reduce the initial population number.For the prevention of primary infection,the first dose can be sprayed at 7 d post blossoming of pear tree,and sprayed for consecutive 2-3 times with an interval of 10-15 d.Prevention and control in summer and autumn should be combined with other diseases at appropriate period,and agents must be sprayed alternately[9].In production,agents with high toxicity and susceptible to pathogens should be selected as far as possible to avoid or delay the generation of resistance to pathogens[10].The timing of drug application should be correctly identified to achieve precise application and drug saving,and agents with different action mechanisms should be used alternately to delay the generation of pathogen resistance and improve disease prevention and control effects[11].The primary infection must be controlled,and the secondary infection must be reduced.

25%Triadimefon WP 1 000 times dilution,40% flusilazole EC 8 000-10 000 times dilution,50% kresoxim-methyl WDG 3 000 times dilution,250 g/L pyraclostrobin EC 2 000 times dilution,430 g/L tebuconazole SC 3 000 times dilution can be sprayed to prevent and control pear powdery mildew,which all have good control effects[12].The quality of spraying should be paid attention,and the spraying should be uniform and thoughtful,so as to give full play to the prevention and control effect of each spraying.