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Disease Cycle and Epidemiology

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Epidemiology

Leucostoma pathogens can invade the host only if the host is predisposed to infection through mechanical wounding or freeze injury. In mature peach and nectarine orchards where L. cinctum is the primary pathogen, many new infections appear at the nodes on one-year-old shoots. The pathogen enters the node through leaf scars or dead buds during the dormant season. Small twigs killed by L. cinctum are pathways by which the pathogen can reach older limbs and initiate cankers that may then result in death of large portions of the tree. Many new infections also take place at pruning wounds, and the fungus most often isolated at these sites is L. persoonii. Generally, pruning cuts are most susceptible if they are made early in the dormant season, but many pruning cuts made in the spring can become infected. At temperatures from 14 to 20 °C (57 to 68 °F), L. persoonii can initiate necrosis, however lesion expansion is limited by host defense reactions. At 2 to 8 °C (35 to 46 °F), initial lesion expansion is slower, however the total branch area colonized is larger and host reaction is more limited. In general, cankers will be smaller as the number of days above 10 °C (50 °F) increases.

The primary inoculum for new infections is conidia. The role of ascospores in the disease cycle remains to be determined. Conidia are most abundant under the cool, moist conditions of late fall and early spring (maxima from November through March) but are present throughout the year, depending upon rainfall. In irrigated regions conidia are potentially present whenever water is available. There are significant correlations between spore capture for L. persoonii and L. cinctum and the number of hours that temperatures are between 10 and 15 °C (50 and 60 °F), the duration of wetness, and the length of time that relative humidity is above 90%. Conidia are resistant to desiccation when contained within the cirrus; however, upon release by water and subsequent drying, most of them die within six hours. The agents involved in dispersal of inoculum have not been established conclusively for these fungi. Short-range dispersal of conidia indicates rain or wind-driven rain as dispersal agents. Localized splash dispersal also may contribute, as does impact sprinkler irrigation. Ascospores are discharged forcibly into the air following rain showers at any time of the year when rainfall occurs, but they are most abundant in late spring and early autumn. Spread of L. persoonii may occur also via infested pruning tools, birds, and possibly by the shot-hole borer (Scolytus rugulosus), although these are minor compared to air, wind-blown rain, and local splash and washing.

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