Visualization of Three Sclerotiniaceae Species Pathogenic on Onion Reveals Distinct Biology and Infection Strategies.

Maikel B F Steentjes,Hilde Coolman,Sander Langebeeke,Olga E Scholten,Sebastian Tonn,Jan A L Van Kan

doi:10.3390/ijms22041865

Maikel B F Steentjes, Hilde Coolman + Show 4 more

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https://doi.org/10.3390/ijms22041865

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Abstract

Botrytis squamosa, Botrytis aclada, and Sclerotium cepivorum are three fungal species of the family Sclerotiniaceae that are pathogenic on onion. Despite their close relatedness, these fungi cause very distinct diseases, respectively called leaf blight, neck rot, and white rot, which pose serious threats to onion cultivation. The infection biology of neck rot and white rot in particular is poorly understood. In this study, we used GFP-expressing transformants of all three fungi to visualize the early phases of infection. B. squamosa entered onion leaves by growing either through stomata or into anticlinal walls of onion epidermal cells. B. aclada, known to cause post-harvest rot and spoilage of onion bulbs, did not penetrate the leaf surface but instead formed superficial colonies which produced new conidia. S. cepivorum entered onion roots via infection cushions and appressorium-like structures. In the non-host tomato, S. cepivorum also produced appressorium-like structures and infection cushions, but upon prolonged contact with the non-host the infection structures died. With this study, we have gained understanding in the infection biology and strategy of each of these onion pathogens. Moreover, by comparing the infection mechanisms we were able to increase insight into how these closely related fungi can cause such different diseases.

Highlights

Onion is an important vegetable crop that is cultivated worldwide, but its production is threatened by pathogens and pests
Botrytis squamosa is the causal agent of onion leaf blight, while Botrytis aclada, Botrytis byssoidea and Botrytis allii cause neck rot, and Sclerotium cepivorum causes onion white rot
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Summary

Introduction

Onion is an important vegetable crop that is cultivated worldwide, but its production is threatened by pathogens and pests. There are three main onion diseases that are caused by species of the fungal family Sclerotiniaceae. The most notorious disease is Botrytis leaf blight, caused by the necrotroph B. squamosa. The fungus was first described nearly 100 years ago, and even today, leaf blight is still a major disease in almost all onion cultivation areas worldwide [1,2,3]. The disease is characterized by small necrotic spots on onion leaves that expand at a later stage leading to blighting of leaves and early leaf senescence, eventually resulting in a reduction of plant growth, bulb yield and quality [4,5]. Compared to neck rot and white rot, the disease cycle of leaf blight is relatively well studied. Infected onion leaves turn necrotic, and on the outside, new conidia are produced that can initiate a new disease cycle [6,7]

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