First and second authors: Department of Plant Pathology, University of Georgia, Athens 30602; and third author: Department of Horticulture, Georgia Station, University of Georgia, Griffin 30223
Pseudosclerotia (infected, mummified fruit) of Monilinia vacciniicorymbosi overwinter on the orchard floor and germinate to produce apothecia in early spring, providing the only source of primary inoculum for mummy berry disease of blueberry. Three experiments were carried out to develop a model for the relative efficacy of mechanical cultivation in reducing the risk associated with primary inoculum. In the first experiment, apothecial emergence from pseudosclerotia buried 0, 1.5, 3, 6, and 10 cm below the soil surface was monitored to determine the critical depth necessary to inhibit emergence. No apothecia emerged from pseudosclerotia buried at depths of ≥3 cm, and the critical depth of burial was determined at 2.6 cm by regression analysis. In the second experiment, pseudosclerotia or plastic beads (used as surrogates for pseudosclerotia) were placed on the soil surface of experimental plots before cultivation with an in-row rotary cultivator, a disc harrow, or a rotary cultivator with each implement operated in a single pass. Vertical distribution profiles of pseudosclerotia or beads in the topsoil were characterized after excavation with a custom-built sampling device. The proportion of pseudosclerotia placed below the critical depth of 2.6 cm was 20.9, 52.6, and 78.6% for the in-row rotary cultivator, the disc harrow, and the rotary cultivator, respectively. For all three implements, vertical distribution profiles of pseudosclerotia and plastic beads were very similar, allowing the latter to be used in subsequent experiments in commercial fields. In the third experiment, two blueberry plantings were surveyed to determine the horizontal distribution of pseudosclerotia on the orchard floor with distance from the crowns of the plants. The greatest frequency of pseudosclerotia occurred between 30 and 40 cm from the plants. Based on measurements of the distance from plants within which different implements can operate, the proportion of pseudosclerotia accessible by cultivation ranged from 58.7% for the disc harrow to 87.2% for the in-row rotary cultivator. Taken together, results from the three experiments indicated that cultivation with a single implement can reduce risk of apothecial emergence by about 50%. More effective risk reductions may be obtained by combining implements that result in deep burial of pseudosclerotia with those that have access to pseudosclerotia near the plants. This was demonstrated by a commercial cultivation method that utilized three passes of different implements and resulted in extensive reshaping of plant beds, placing 88.2% of beads below the critical depth of 2.6 cm.