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First Report of Powdery Mildew Caused by Erysiphe heraclei on Celery in North America
Abstract:
Since 1994, a fungal disease has been affecting celery (Apium graveolens) grown for seed in the central coast region of California. White, ectophytic mycelia and conidia, characteristic of a powdery mildew, were present on compound umbels, including the involucre, rays, raylets, flowers, and fruits. Celery with powdery mildew had twisted flower petals, and the disease possibly contributed to premature senescence and drying of the fruits. Powdery mildew was not observed on leaves. Mycelial growth was effused and amphigenous and had lobed appressoria. Conidiophores were straight, and cylindrical foot cells were followed by a longer cell and one or two shorter cells. Conidia were produced singly and were 36 to 44 × 11 to 16 μm. Fibrosin bodies were not observed. Germ tubes were located at the ends of conidia and formed lobed appressoria. Cleistothecia were not present. Based on these characteristics, the fungus was identified as Erysiphe heraclei (1). In California, parsley (Petroselinum crispum) is a host of E. heraclei (4), and parsley is grown in the same region as celery seed crops. To determine if parsley was an alternative host of the celery powdery mildew, infected celery umbels were gently pressed onto adaxial surfaces of leaves on 2-month-old greenhouse grown parsley cvs. Italian Plain and Triple Curled. Inoculated plants were incubated 48 h in a moist chamber at 22°C, then transferrred to a greenhouse at 22 ± 2°C / 16 ± 2°C day/night temperatures, 75% relative humidity, and natural light. After 12 to 14 days, powdery mildew was observed on both parsley cultivars. Uninoculated control plants did not develop the disease. This is the first report of powdery mildew on celery in North America. Previous reports list E. heraclei on celery in Chile, the former Czechoslovakia, France, Iraq, Italy, and the former USSR (Georgia region). In the U.S., E. heraclei is reported on carrot (Daucus carota) (2), parsley (4), meadow parsnip (Zizia aptera and Z. aurea) (3), and now celery. References: (1) H. J. Boesewinkel. Bot. Rev. 46:167, 1980. (2) D. F. Farr et al. Fungi on Plants and Plant Products in the United States. American Phytopathological Society, St. Paul, MN. (3) K. A. Hirata. Host Range and Geographic Distribution of the Powdery Mildew Fungi. Jpn. Scientif. Soc. Press, Tokyo. (4) S. T. Koike and G. S. Saenz. Plant Dis. 78:1219, 1994.
Insights
Powdery mildew, identified as Erysiphe heraclei, has been confirmed on celery seed crops in California. This study demonstrates that parsley is a susceptible host, indicating a potential for disease spread in the region.
Area of Science:
- Plant Pathology
- Mycology
- Agricultural Science
Background:
- A fungal disease, characterized by white, ectophytic mycelia and conidia, has affected celery (Apium graveolens) seed crops in California since 1994.
- The symptoms include infection of flower parts, leading to twisted petals and potential premature fruit senescence, with no observed leaf infection.
- The causal agent was morphologically identified as Erysiphe heraclei, a known pathogen of other Apiaceae family members.
Purpose of the Study:
- To identify the causal agent of powdery mildew on celery seed crops in California.
- To determine if parsley (Petroselinum crispum), a common crop in the region, serves as an alternative host for the celery powdery mildew pathogen.
- To document the first report of powdery mildew on celery in North America.
Main Methods:
- Morphological characterization of the fungus on infected celery umbels, including conidia size and appressoria structure.
- Inoculation of greenhouse-grown parsley cultivars ('Italian Plain' and 'Triple Curled') with infected celery material.
- Incubation of inoculated parsley plants under controlled conditions (moist chamber, greenhouse temperature, and humidity) to observe disease development.
Main Results:
- The fungus was identified as Erysiphe heraclei based on its morphological characteristics.
- Inoculation experiments confirmed that parsley cultivars 'Italian Plain' and 'Triple Curled' are susceptible to the celery powdery mildew pathogen.
- Powdery mildew symptoms appeared on inoculated parsley leaves 12–14 days post-inoculation, while control plants remained healthy.
Conclusions:
- Erysiphe heraclei is confirmed as the pathogen causing powdery mildew on celery seed crops in California.
- Parsley is identified as a susceptible alternative host for E. heraclei, posing a risk for disease establishment and spread in regions where both crops are cultivated.
- This study represents the first report of powdery mildew on celery in North America and expands the known host range of E. heraclei in the United States.
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