Pear Scab model (Venturia pyrina)

The pear scab fungus forms ascospores just like apple scab, but can also overwinter on 1- to 3-year-old wood. These wood scab cankers produce conidiospores starting before bud break, and lasting till summer. Pear Scab lesions on leaves are mainly found on the lower side of leaves, in low numbers, even where a high number of fruits is showing symptoms.

Typically (on Conference pears) the first lesions on fruits are found immediately after bloom, resulting from pre-bloom infections. Most of these fruits drop in the weeks after. Later symptoms become only visible during the last month or weeks before harvest, and tend to increase during storage.

In pear orchards where no scab lesions on fruits occurred in the previous year, scab management is easy. A limited spray schedule is sufficient to keep the disease on a minimal, undetectable level. We assume in these situations ascospores are the only inoculum. The RIMpro model for primary (ascospore) infections of apple scab can be followed, but the biofix date should be set for green tip of pear.

In pear orchards where fruits showed scab lesions after storage, effective scab management immediately becomes more difficult. We assume in these situations the inoculum is mainly conidia produced by lesions on wood and twigs. Already at the beginning of the vegetation season the ascosporic inoculum is outnumbered by wood scab conidia.

Growers find the control of pear scab most troublesome in vigorous trees and orchards. The incidence of scabbed fruits is well correlated to growth level of individual pear trees. Reduction of shoot growth level by means of root pruning or reduction of fertilisation, reduced fruit scab incidence in field trials.

The Model

You must set the date of green tip, and the date of full bloom for your pear orchard under ‘local parameters’ for each station.

RIMpro offers two models for fruit infections by Pear scab.

The ‘Basic’ static model, that just combines wetness duration with temperature, and indicates slight, medium and high germination chances for spores, as a Mills table does for apple scab. The calculations are based on research data published by Spotts 1991, 2010, Villalta 2000.

The dynamic model (Fig below) includes the quantitative availability and splash distribution of conidiospores from wood scab cankers, advanced infection simulation, and gradual reduced infection efficiency due to ontogenetic resistance of the fruits.

The lower graph “Wood Scab’ estimates the production of conidiospores by the wood scab cankers based on detailed work by Saccas in 1944. The first conidiospores are produced before the date you set as bud break. The spores are rain-splash-distributed from the wood scab cankers over other parts of the tree.

In the white in the middle graph “Potential Infection” indicates the germinating spores. The red infection line indicates number of spores that succeeded in infecting the plant surface and determines the potential severity of the infection

The red infection lines in the upper graph “Effective Infection” includes the susceptibility of fruits of the main variety Conference. After bloom the susceptibility of the fruits gradually declines. We found in our field trials over the years that on Conference fungicide applications after mid-June did not further contribute to the control of scab on fruits. Only in orchards with a very high inoculum later treatments might add little to efficacy. The pink area shows the relative susceptibility of the fruits. The infection risk for fruits in the upper graph is calculated from the potential severity as calculated in the middle graph * the relative susceptibility of the fruits. This makes that the potential infections calculated from the beginning of June are estimated less important for fruit infections as the fruits mature.

The use of the Model for scab management

The model is still experimental.

The model holds all public available information on pear scab, and the results of trials by the Dutch Pear Scab Working Group 2010-2020.

Testing the model under field conditions has proved difficult because of the strong effect of disease pressure on the results, and the limited efficacy of the fungicides available for organic pear production.

From the field results 2017-2020 obtained in organic pear orchards in The Netherlands and Belgium we can however conclude that:

1- In organic pear orchards where in previous year’s harvest, after storage, less than 5% of the fruits showed scab symptoms, a curative schedule based on the use of sulphur, bicarbonate and lime sulfur, aimed at infection moments as indicated by the model was effective.

2- In organic pear orchards with more than 5% scabbed fruits post-storage, the curative schedule needed support by preventive low-rate copper applications to be reasonable effective.

The key for effective scab management on pear is a low inoculum and termination of shoot end of June. With the effective chemical fungicides available in integrated production, the control of Pear scab is seldom a problem. Under these conditions we think the model is a good guide for decisions on fungicide applications.

For organic production, with curative fungicides that have a small times slot in the infection biology for optimal efficacy, accurate timing, an appropriate dose, and excellent sprayer calibration and application techniques is required.

In orchards with an increased disease pressure, either organic, or integrated, effective control is a challenge. This experience goes back to the early 20^th century (Kienholz 1937). The infections as indicated by the model will still be the key moments in disease control, but under practical conditions the curative fungicide applications should be completed by protective treatments shortly before the rain starts. Because of the high spore pressure, infections between mid-June and July indicated at having minimal importance in the graph “Effective Infection”, should be taken serious too.