Leaf Scorch Epidemics Reduce Vegetative Growth and Fruit Yield of `Kent' Strawberry

Abstract

<jats:p>The relationship between severity of leaf scorch epidemics, caused by <jats:italic>Diplocarpon earlianum</jats:italic>, and components of vegetative growth and fruit yield was examined in `Kent' strawberry (<jats:italic>Fragaria</jats:italic> ×<jats:italic>ananassa</jats:italic> Duch.). Plants were treated in July with six densities of initial inoculum of the pathogen, and severity of leaf scorch was assessed at 2-week intervals from late July to late October. After an analysis of vegetative growth in late October, plants were overwintered in the field or grown in a greenhouse, and later assessed for yield components. Relationships between area under the disease progress curve (AUDPC) and plant growth and yield components were examined by regression analysis. Significant negative linear relationships were found between AUDPC values and number of green leaves, leaf area, leaf dry weight, crown number, crown dry mass, and root dry weight. Significant negative relationships were also found between AUDPC values and number of flowers and inflorescences, and total and marketable berries in the subsequent season, in plants maintained in the field or in the greenhouse. Mean berry weight was not significantly affected. Reduction in the number of crowns in plants affected by leaf scorch was a major factor limiting the yield of diseased plants. In an analysis of regrowth at seven weeks after fruit harvest, a significant negative linear relationships was found between AUDPC values and each growth component except crown dry weight. Collectively, the data provide a rationale to optimize timing of treatments, such as chemical fungicides or microbial agents, to control leaf scorch in August, September and October and thereby promote berry yield in the subsequent season.</jats:p>

Estimation of the impact of leaf scorch on photosynthesis and "physiological-lesion" size in strawberry

“…When taken in the context of previous results (Mutisya et al 2005a(Mutisya et al , 2005bKerkhoff et al 1988b), our data indicate that a reduction in yield could be expected when visual leaf-scorch severity exceeds 40% and is a combined effect of the visually damaged portion of the leaflet and a reduction in A and WUE in the area surrounding the visible lesion. The magnitude of the reduction, however, is dependent on additional factors as discussed above.…”

“…Of these factors, the difference between β values in our experiment, although relatively small, may be attributed to differences in the mean leaf age between the greenhouse and field studies. Mutisya et al (2005b) reported significant yield reductions in their experimental planting of 'Kent' when leafscorch severity reached approximately 45%. However, Kerkhoff et al (1988a) showed that plants defoliated manually during the period of flower-bud initiation in the prior growing season had significant reductions in the total number of flowers and fruit when defoliation exceeded 66% (i.e., two of the three leaflets removed from each leaf).…”

“…Also, it should be noted that many factors affect the relationship between yield and photosynthesis, including environmental and climatic conditions, nutrient and water availability, rate of CO 2 uptake, temporal assimilate allocation, canopy structure, age of the crop, and crop load (Rossing et al 1992;Francesconi et al 1996). The yield losses reported by Mutisya et al (2005aMutisya et al ( , 2005b were the result of infections that occurred prior to and during the period of flower-bud formation. Similar levels of infection occurring during other growth periods (e.g., after flower-bud formation) may not have the same effect on yield.…”

“…However, Mutisya et al (2005aMutisya et al ( , 2005b found that a number of vegetative components, including the size and number of leaves and crowns, were reduced proportionally with disease severity as disease progressed into the fall on 'Kent' strawberry plants inoculated with D. earlianum conidia in July. In the following spring, they found that the number of flowers and inflorescences and, subsequently, the yield were similarly reduced.…”

“…Significant yield reductions were found when dis-ease severity reached approximately 45% at the end of the growing season in their experimental plantings. In their most heavily affected plots, yield was reduced by 27% (Mutisya et al 2005b).…”