Assessing Inoculum of Soilborne Plant Pathogens: Theory and Practice in Decision-Making for Soil Disinfestation

Inoculum of a pathogen is its biomass available for infection. Inoculum density (ID), usually expressed per unit of volume or (dry) weight of soil, is the common measure to evaluate the effect of soil disinfestation. Interpretation of ID of a given pathogen depends on the sampling and detection procedures. For example, soil drying and sieving selects for relatively large surviving propagules, and ID only reflects the density of these surviving structures. Factors affecting the reliability of the prediction of future damage based on ID include: no more than one type of surviving propagule; absence of a saprophytic phase; strong sensitivity to fungistasis; high survival capacity; and availability of validated sampling and detection methods. The efficacy of soil disinfestation is often determined rapidly after finishing the disinfestation treatment. If that is the case, certain detection methods may lead to false positives. The perspective of a given soil disinfestation method not only depends on the relationship between ID and disease incidence/severity, but also on the initial ID prior to treatment. For a given soil disinfestation method there is, theoretically, a threshold initial ID, above which the method leads to a final ID that is higher than the damage threshold; if no other methods are available, a switch to resistant crops may be the only other control alternative. In soil disinfestation experiments, scientists can choose between a natural level of infestation or introduce inoculum. In the first case, spatial variation of inoculum has to be dealt with, while in the second case, care has to be taken that the introduced inoculum must have the same properties such as persistence and germinability as the inoculum occurring naturally.