Brown rot, caused by Monilia spp. (Monilia laxa, Monilia fructigena and Monilia fructicola) belong to the most destructive diseases on stone fruits through Europe. Especially in cherry production the economic impact is significant.
Symptoms of the brown rot disease are the blight of the cherry blossom as well as the green tip of twigs due to the penetration of the pathogen into the open blossom through the stigma of pistils or anthers. This usually results in wilting of the whole part of an one-year-old twig. The leaves start to hang down, later they become brown and rigid, but usually do not fall down on the soil, they remain on the tree until the spring of the next year. Sometimes, especially under humid conditions droplets of gum are visible, which are symptoms of colonisation of the fungus as well as the established cankers. The infected fruits are covered by putrefactive spots, from which warty sporodochia (hyphe) with conidia of the "summer" form appear. Additionally, in late autumn and winter, the fungus produces sporodochia of the "winter" form on infected twigs. With time, severely affected fruits become mummified. The mycelium growing in such mummies gradually aggregates into sclerotia. Such fruits remain on the tree during winter.
Temperature and wetness duration are important environmental factors, determining the infection incidence of M. laxa on sweet cherry blossoms. Monilia laxa is well adapted to the relatively low temperatures during spring and cause infections at temperatures as low as 5°C within a very short period of wetness duration. The infection of the active bloom trough the stima does not need very much leaf wetness. Leaf wetness is only needed for germination of the conidia. Therefore infection of the young fruit needs longer leaf wetness periods. To infect the young fruit an appressoria has to be formed and free moisture is needed to build up the pressure to form the infection peg to enter the epidermis cell. With maturity of the fruit small scars on the fruits allow an infection without infection peg again and the needed leaf wetness duration becomes shorter again.
No sporulation is possible if relative humidity does not overcome 85% rel. humidity. The M.laxa model in fieldclimate.com calculates weak and severe infections. Weak infections are modelled for conditions to infect highly susceptible stages like flowers and wounded fruits close to maturity. To infect this tissue very short leaf wetness periods have shown to be sufficient. Several studies, for example:" Phenological Analysis of Brown Rot Blossom Blight of Sweet Cherry Caused by Monilinia laxa" (L. Tamm, Chr. E. Minder, and W. Flickiger; 1994) or "Effects of wounding, fruit age and wetness duration on the development of cherry brown rot in the UK." ( X.-M. Xu*, C. Bertone and A. Berrie ;2003) confirmed low wetness periods needed for infections . Further on, immature fruits are more resistant for infections, but close to maturity they are becoming mor susceptible and conditions for severe infections are given.
Springtime disersal patterns of Monilinia laxa conidia in arpicot, peach, prune and almond trees. Canadian Journal of Botany (1974), 52: 167-176
Fieldclimate Modeling: It is probably that the time, needed for infection during bloom has to be shortened. Therefore the model is shorting infections down in the area of 2000 to 4800 degree hour above 5°C.
The first graph shows a model for Styria in the year 2010, which indicates an weak infection at the 06th of April. At this time the cherry tree starts to bloom. This infection can already lead to a latent infections of the fruits, which would cause severe damages. Late blossom will be completely destroyed by infections on mid to end of April. Severe infections are determined on the 6th of April due to long leaf wetness periods and temperatures above 5 to 15°C.