Abstract
Development of resistant cultivars for Fusarium basal rot (FBR), a fungal disease caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), is a major breeding goal for onion breeding programs around the world. Currently, the selection of FBR-resistant bulbs is based on a visual interval or category scale, which is an entirely subjective method. This study aimed to develop an objective approach using digital image analysis to quantify symptom development in the basal plate of dormant bulbs. Digital image analyses were performed after artificially inoculating dormant bulbs of eighty-five United States Department of Agriculture (USDA) Allium cepa accessions with a virulent FOC isolate, ‘CSC-515’. An analysis with confocal microscopy identified bright blue–green autofluorescence from FOC-infected tissue, effectively differentiating diseased from healthy tissue. Visual scoring of the FBR symptom was aided by stereo fluorescence microscopic images captured using a green fluorescence protein dual filter to quantify accurately FBR severity in the basal plate tissue. An automatic stepwise image segmentation method was developed that was relatively more accurate than a visual estimation. This method exhibited comparable reliability and precision to visual scoring, but it tended to underestimate FOC infection. To our best knowledge, this is the first comprehensive study to investigate the potential use of image analysis as a viable alternative to conventional visual scoring for FBR symptom development. This method could be used for developing resistant cultivars for onion breeding programs in the near future.
Highlights
IntroductionFusarium basal rot (FBR) is one of the most devastating diseases impacting onions across the world
Bright autofluorescence was observed in the digital images of FOC-inoculated onion basal plate tissue captured via confocal microscopy (Figure 3a)
The autofluorescence of the FOC‐infected basal plate tissue, which fell in the blue–green region of the white light spectra, helped to identify the right optical filter of the stereo fluorescence microscope for scoring FOC infection with digital images of the basal plates of the plant introduction (PI) accessions
Summary
Fusarium basal rot (FBR) is one of the most devastating diseases impacting onions across the world. FBR, which affects onion production in the tropical and sub-tropical parts of the world, is caused by the soil-borne saprophytic fungus Fusarium oxysporum f.sp. The characteristic visible symptoms of FBR can be observed at all plant growth stages of onion that include damping-off of seedlings, chlorosis and necrosis of leaves, and discoloration of the basal plate tissue [1]. Even though all crop growth stages are susceptible to this disease, early detection of this disease is not always possible as a minor infection in the basal plate tissue often goes undetected. A substantial loss of marketable bulbs occurs because the disease appears to reduce bulb weight and increases bulb rot leading to secondary infections with a favorable environment during storage [2,3]
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