High Incidence of Preharvest Colonization of Huanglongbing-Symptomatic Citrus sinensis Fruit by Lasiodiplodia theobromae (Diplodia natalensis) and Exacerbation of Postharvest Fruit Decay by That Fungus

Abstract

Huanglongbing (HLB), presumably caused by the bacterium "Candidatus Liberibacter asiaticus," is a devastating citrus disease associated with excessive preharvest fruit drop. Lasiodiplodia theobromae (diplodia) is the causal organism of citrus stem end rot (SER). The pathogen infects citrus fruit under the calyx abscission zone (AZ-C) and is associated with cell wall hydrolytic enzymes similar to plant enzymes involved in abscission. By means of DNA sequencing, diplodia was found in "Ca. Liberibacter asiaticus"-positive juice from HLB-symptomatic fruit (S) but not in "Ca. Liberibacter asiaticus"-negative juice. Therefore, the incidence of diplodia in fruit tissues, the impact on HLB-related postharvest decay, and the implications for HLB-related preharvest fruit drop were investigated in Hamlin and Valencia oranges. Quantitative PCR results (qPCR) revealed a significantly (P < 0.001) greater incidence of diplodia in the AZ-C of HLB-symptomatic (S; "Ca. Liberibacter asiaticus" threshold cycle [C T ] of <30) than in the AZ-C of in asymptomatic (AS; "Ca. Liberibacter asiaticus" C T of >30) fruit. In agreement with the qPCR results, 2 weeks after exposure to ethylene, the incidences of SER in S fruit were 66.7% (Hamlin) and 58.7% (Valencia), whereas for AS fruit the decay rates were 6.7% (Hamlin) and 5.3% (Valencia). Diplodia colonization of S fruit AZ-C was observed by scanning electron microscopy and confirmed by PCR test and morphology of conidia in isolates from the AZ-C after surface sterilization. Diplodia C T values were negatively correlated with ethylene production (R ‫؍‬ ؊0.838 for Hamlin; R ‫؍‬ ؊0.858 for Valencia) in S fruit, and positively correlated with fruit detachment force (R ‫؍‬ 0.855 for Hamlin; R ‫؍‬ 0.850 for Valencia), suggesting that diplodia colonization in AZ-C may exacerbate HLB-associated preharvest fruit drop. C itrus huanglongbing (HLB; also known as citrus greening) is one of the most devastating diseases of citrus and has spread throughout the major citrus-producing regions in Asia, Africa, and the Americas, causing great losses for the citrus industry worldwide (1). HLB is associated with "Candidatus Liberibacter" spp., which are Gram-negative, phloem-limited bacteria (2, 3). The Asian form of HLB is currently present in the United States, and the causal pathogen, bacterium "Candidatus Liberibacter asiaticus," was first confirmed in Southeast Florida in 2005 (1) and now is prevalent in all Florida citrus-growing areas in the state. HLB disease causes substantial economic loss by debilitating the productive capacity and shortening the life span of infected trees, promoting fruit drop and reducing fruit quality (4). Yield reduction can reach 30 to 100%, depending on the proportion of the canopy affected and the age of the trees during inoculation (1, 5).HLB-diseased citrus plants develop a multitude of symptoms. Leaf symptoms include chlorosis, which commonly appears in an asymmetric pattern referred to as "blotchy mottle" (4). Chlorosis may typically appear on a single branch and subse...

Endophytic Fungi of Citrus Plants

“…The agent of citrus black spot (CBS) Phyllosticta citricarpa, also known under the teleomorph name Guignardia citricarpa (Dothideomycetes, Botryosphaeriaceae), is one of the most noxious pests of these crops in subtropical regions, and it is subject to phytosanitary restrictions by the European Union and the United States. The employment of biomolecular methods has provided substantial support to the distinction between pathogenic isolates, typically slow-growing in axenic cultures and producing a yellow halo on oatmeal agar, and non-pathogenic isolates, which are morphologically reported to exacerbate pre-harvest fruit drop and post-harvest fruit decay in plants of Citrus sinensis hit by the huanglongbing syndrome [39].…”

Recovery of pectic hydrocolloids and phenolics from huanglongbing related dropped citrus fruit

“…There is no known cure, only stop gap, temporary and often poorly performing fixes. Unfortunately, this disease causes pre‐harvest fruit drop, drastically reducing yields, as well as increased rejects at processing plants. During the 2013–14 harvesting season, 31% of the early/mid‐season fruit and 25% of the late‐season crop dropped from the trees.…”

Natural plant volatiles as an alternative approach to control stem‐end rot in avocado cultivars

“…Endophytically, these fungi live asymptomatically and occasionally symptomatically within the healthy tissue of the host (Faeth & Fagan, ). In South Africa, L. theobromae has been recognized as one of the species of Botryosphaeriaceae that affects avocado (Darvas & Kotzé, ) and other subtropical fruits such as mango (Perumal, Sellamuthu, Nambiar, & Sadiku, ; Syed, Mansha, Khaskheli, Khanzada, & Lodhi, ), banana (Nath, Solanky, Mahatma, & Madhubala, ; Twumasi, Ohene‐Mensa, & Moses, ), papaya (Cavalcante et al., ; Netto et al., ) and citrus (Zhang et al, ; Zhao, Bai, McCollum, & Baldwin, ).…”