Rafaela G. Ruschel's research while affiliated with São Paulo State University and other places

Macrophomina phaseolina, the causal agent of charcoal rot in strawberry, induces plant wilting and collapse. The pathogen survives through the production of microsclerotia in the soil and in strawberry debris. However, its management is difficult, and the disease has become an increasing problem for the strawberry industry. Physical, cultural, and chemical alternatives for integrated management of charcoal rot were evaluated in laboratory and field trials during the 2017-18 and 2018-19 strawberry seasons. In a laboratory trial, M. phaseolina microsclerotia were subjected to heat treatment and germination was inhibited at 52, 56, 80, and 95°C after 30, 10, 1 and 0.5 min of exposure, respectively. In infected strawberry crowns, microsclerotial viability was reduced after 5 min, regardless of temperature, whereas in the field, reduction was observed after 1 min. In field trials, charcoal rot incidence of inoculated strawberry plants transplanted into white-striped plastic-mulched beds was reduced to 20.8%, compared to 60.8% for plants grown in the black plastic mulch. On commercial farms, crop residue removal from infested areas reduced the M. phaseolina population in the soil but did not decrease charcoal rot incidence. Moreover, M. phaseolina propagule densities in the soil and in strawberry debris was reduced by fumigant application at crop termination but surviving propagules allowed the population to increase over the summer. Furthermore, pre-plant fumigation with metam potassium reduced soil population and charcoal rot incidence. Overall, the adoption of integrated approaches such as physical, chemical, and/or cultural methods played a significant role in reducing M. phaseolina inoculum and contribute to control of the disease in areas with high disease pressure.

Pestalotiopsis-like species have been reported affecting strawberry worldwide. Recently, severe and unprecedented outbreaks have been reported in Florida commercial fields where leaf, fruit, petiole, crown, and root symptoms were observed, and yield was severely affected. The taxonomic status of the fungus is confusing since it has gone through multiple reclassifications over the years. Morphological characteristics, phylogenetic analyses, and pathogenicity tests were evaluated for strawberry isolates recovered from diseased plants in Florida. Phylogenetic analyses derived from the combined ITS, β-tub, and tef1 regions demonstrated that although there was low genetic diversity among the strawberry isolates, there was a clear separation of the isolates in two groups. The first group included isolates recovered over a period of several years, which was identified as Neopestalotiopsis rosae. Most isolates recovered during the recent outbreaks were genetically different and may belong to a new species. On PDA, both groups produced white, circular, and cottony colonies. From the bottom, colonies were white to pale yellow for Neopestalotiopsis sp. and pale luteous to orange for N. rosae. Spores for both groups were five-celled with three median versicolored cells. Mycelial growth and spore production were higher for the new Neopestalotiopsis sp. isolates. Isolates from both groups were pathogenic to strawberry roots and crowns. However, the new Neopestalotiopsis sp. proved more aggressive in fruit and leaf inoculation tests, confirming observations from the recent outbreaks in commercial strawberry fields in Florida.

Pestalotiopsis spp. have been known to cause strawberry fruit rot. In recent reports from Florida and other strawberry producing areas, the fungus was associated with root and crown rots. The taxonomic status is confusing as two novel genera, Neopestalotiopsis and Pseudopestalotiopsis, have been described. During the 2018-19 season, a severe outbreak was observed in two Florida fields where leaf, fruit, petiole, crown, and root symptoms were present. Currently, there are no fungicides labeled to control this disease in the U.S. Therefore, morphological and molecular characteristics, pathogenicity, and fungicide sensitivity of isolates from Florida were evaluated. Colonies were white, circular, and cottony on the upper side. On the bottom, root, crown, and petiole isolates were pale luteous to orange; and leaf and fruit isolates were white to pale yellow. Optimum temperatures for mycelial growth and sporulation were 25 and 30°C, respectively. In pathogenicity tests, all isolates were able to produce root and crown rot symptoms. However, on fruit, disease incidence and severity were higher with leaf and fruit isolates. Thirty-eight isolates were screened in vitro for their sensitivity to azoxystrobin, captan, fluopyram, penthiopyrad, flutriafol, tetraconazole, fludioxonil, and thiophanate methyl. Only captan and fluodioxonil significantly inhibited mycelial growth. Moreover, resistance to QoI fungicides was confirmed by the presence of the G143A mutation in the cytb gene. Molecular studies are currently being performed to determine the taxonomic status of Florida isolates.