Academic literature on the topic 'Almond; fungal disease, anthracnose; epidemiology'

Almond anthracnose caused by Colletotrichum spp. has been described as one of the most important diseases of this nut crop in the main almond-growing regions worldwide, including California, Australia and Spain. Currently, almond anthracnose is considered a re-emerging disease in the countries across the Mediterranean Basin due to the shift of plantations from the original crop areas to others with climatic, edaphic and orographic conditions favoring crop growing and yield. The pathogen mainly affects fruit at the youngest maturity stages, causing depressed, round and orange or brown lesions with abundant gum. The affected fruits can fall prematurely and lead to the drying of branches, causing significant economic losses in years of epidemics. This review aims to compile the current knowledge on the etiology, epidemiology and management of this disease.

Almond, [Prunus dulcis (synonym Prunus amygdalus)] planted on approximately 595,000 acres (240,797 ha), is California's largest acreage tree crop. California's Central Valley accounts for nearly 100% of the U.S. domestic production of almonds. Integrated pest management (IPM) programs that integrate cultural practices and pest and disease monitoring with selective controls have improved plant protection in almond. Methods of orchard floor management and their effects must also be taken into account. Minimizing dust reduces mites while harvesting earlier and the destruction of overwintering refugia are cultural practices that reduce worm damage. Improved methods for field sampling and monitoring have reduced the need for pesticide applications while improving timing and effectiveness of needed crop protection sprays. Selective controls have further reduced the impact on nontarget species. Augmentative parasite releases have also helped manage navel orangeworm (Ameylois transitella). Effective use of new selective fungicides will require precise application timing and greater knowledge of diseases and resistance management. A better understanding of disease life cycles leading to improved monitoring of the fungal diseases, shothole (Wilsonomyces carpophilus), almond scab (Cladosporium carpophilum), and anthracnose (Colletotrichum acutatum) have reduced fungicide applications. Future challenges include the potential loss of effective pest control products, the need to continually develop improved utilization strategies, and maintaining economic sustainability.

Almond anthracnose, caused by Colletotrichum spp., is a reemerging disease in Spain. To date, little research has been conducted on the factors affecting this disease development. In this study, the effects of cultivar, fruit wounding and maturity, leaf age, fungal isolate, and temperature on almond infection by Colletrotrichum spp. were evaluated under laboratory-controlled conditions. Inoculations were performed using conidial suspensions of Colletrotrichum acutatum or C. godetiae. Disease severity was higher in wounded than in unwounded fruit. Based on observations of inoculated fruit, Ferraduel and Nonpareil were the most tolerant cultivars, while Tarraco and Penta were the most susceptible cultivars. Four categories of susceptibility (highly susceptible, susceptible, moderately susceptible, and resistant) were distinguished by using the cluster analysis statistical approach. Differences in susceptibility between young and old leaves were observed, but Nonpareil was consistently the most tolerant cultivar. Significant differences in virulence between C. acutatum and C. godetiae were observed in inoculated fruit, with C. acutatum being the most virulent. Disease development was more severe when inoculations were performed at the fruitlet stage or when the fruit were incubated at approximately 25°C, with respect to other maturity stages and temperatures evaluated. Natural fruit infections were also assessed. Cultivar susceptibility data were compared between laboratory tests and field observations. A significant positive linear correlation was obtained between the susceptibility of the common cultivars evaluated under the two conditions.

Modulation of pH within the host during infection of almond by the anthracnose pathogen Colletotrichum acutatum was studied using confocal scanning laser microscopy and the dual emission fluorescence indicator SNARF-1. This highly sensitive method allowed visualization of the spatial distribution of localized pathogen-induced pH modulation within and in proximity to fungal infection structures in host tissue at the cellular level. Ratiometric measurement of fluorescence at two emission wavelengths and in situ calibration allowed the quantification of pH ranges. After incubation of leaf epidermal tissue with SNARF-1, distinct alkaline (pH 8 to ≥9), red-spectrum (650 nm wave length) fluorescent zones developed as partial or complete halos around many fungal appressoria and in infection vesicles at 24 to 36 h after inoculation. In samples taken after 48 to 72 h, colonizing hyphae in the biotrophic phase and subsequently in the necrotrophic phase were also emitting the red fluorescence that extended into the surrounding host tissue, as also verified by depth analyses. Host epidermal cells were intact and apparently alive during the fungal alkalization process, with no visible disruption of cell structure. Generally, the pH of epidermal cells in noninoculated samples or in areas away from the infection in inoculated samples was lower than pH 7 with green (i.e., 500 to 550 nm wave length) fluorescence detected. Using standard electrodes, a significant increase in pH and ammonia concentration in leaf and fruit tissue was also measured but only at advanced stages of disease. In contrast, hyphae of the pathogen Alternaria alternata were mostly acidic and no change in fluorescence was found inside invaded host cells. The sequence of events in the C. acutatum–almond interaction includes penetration, production of ammonia by C. acutatum, and subsequent pH modulation within almond epidermal tissue to an alkaline environment that leads to further colonization of the host.

Colletotrichum spp. are broad-range pathogens, meaning that many species can infect a single host and a single species can infect diverse hosts. For example, Colletotrichum acutatum J.H. Simmonds affects a wide range of crops, causing disease symptoms on apple, almond, anemone, citrus, lupin, peach, pecan, strawberry, and others, whereas Colletotrichum gloeosporioides Penz. (Penz. & Sacc.) can affect many of the previous hosts as well. Anthracnose is one of the major fungal diseases of strawberry occurring worldwide. In Israel, the disease is caused primarily by the species C. acutatum. The pathogen causes irregular leaf spot, bud rot, petiole and stolon necrosis, and black spot on fruit. The pathogen is most destructive when it causes root necrosis and crown rot, which usually kill the plants in nurseries and transplants in the field. To maintain a disease-free crop, nuclear and foundation stock material, as well as field nurseries, must be routinely monitored and tested for presence of the pathogen. Strawberry cultivation using plasticulture as an overhead cover of the crop can significantly control anthracnose disease incidence by reducing inoculum spread and infection, both in nurseries and in production fields. C. acutatum from strawberry can survive on several cultivated plant species, such as pepper, eggplant, tomato, bean, and weed species, without causing disease symptoms. This indicated that they may serve as a potential inoculum reservoir for strawberry infection between seasons. Although C. acutatum survives in soil under certain conditions, no specific resting structures have been observed indicating that the pathogen does not behave as a typical soilborne fungus.

Fungal pathogens are a major constraint affecting the quality of pomegranate production around the world. Among them, Alternaria and Colletotrichum species cause leaf spot, fruit spot or heart rot (black rot), and fruit rot (anthracnose) or calyx end rot, respectively. Accurate identification of disease-causing fungal species is essential for developing suitable management practices. Therefore, characterization of Alternaria and Colletotrichum isolates representing different geographical regions, predominantly Maharashtra—the Indian hub of pomegranate production and export—was carried out. Fungal isolates could not be identified based on morphological characteristics alone, hence were subjected to multi-gene phylogeny for their accurate identification. Based on a maximum likelihood phylogenetic tree, Alternaria isolates were identified as within the A. alternata species complex and as A. burnsii, while Colletotrichum isolates showed genetic closeness to various species within the C. gloeosporioides species complex. Thus, the current study reports for the first time that, in India, the fruit rots of pomegranate are caused by multiple species and not a single species of Alternaria and Colletotrichum alone. Since different species have different epidemiology and sensitivity toward the commercially available and routinely applied fungicides, the precise knowledge of the diverse species infecting pomegranate, as provided by the current study, is the first step towards devising better management strategies.

A new leaf spot disease of almond (Prunus dulcis [Mill.] D. Webb) was observed in California in the late 1980s and was first associated with severe defoliation in the mid-1990s (1). Orchards in areas with frequent summer dews, high humidity, and little air movement sustained severe defoliation, resulting in yield losses often exceeding 50%. Symptoms occur only on leaf blades in late spring and summer. Lesions develop as small, circular, tan spots 1 to 3 mm in diameter that may enlarge to 5 to 20 mm in size. Semicircular lesions frequently develop along the leaf margins and tips. The centers of mature lesions become black with fungal sporulation. The fungi isolated from the margins of sporulating and non-sporulating lesions were identified as three species in the Alternaria alternata complex: A. alternata, A. arborescens, and A. tenuissima (2,3). Cultures grown in the dark on potato dextrose (PDA) or potato-carrot agar are grayish white to olivacious green in the former two species and dark gray and wooly in the latter species. On 5% PDA, cultures of all three species produced catenulate dictyospores that were granular to punctate (-verrucose), pale yellowish to brown or black, and had visible apical and basal pores. Conidial morphology depended on chain position; apical conidia ranged from ovoid to ellipsoid, whereas basal conidia were elliptical to obclavate. Average conidial dimensions of A. alternata and A. arborescens ranged from 20 to 28 × 8 to 10 μm. Conidia of A. alternata were produced in acropetal succession in branching chains on single, short suberect conidiophores. A. arborescens produced conidia similarly but mostly in dichotomously branching chains on short to long conidiophores. Average conidial dimensions of A. tenuissima ranged from 20 to 34 × 8 to 12 μm and they were produced in simple chains with one or two branches forming occasionally. In preliminary studies, the optimum temperature for mycelial growth on PDA for all three species ranged from 24 to 28°C. Fifty mature leaves on each of four 7- or 8-year-old almond cv. Butte trees were inoculated at 2- to 3-week intervals from mid-spring through summer in 1999 and 2000. Leaves were sprayed with aqueous suspensions containing 105 conidia per milliliter for one isolate each of A. alternata and A. arborescens and two isolates of A. tenuissima or with sterile distilled water. The shoots were covered for 72 h with plastic-lined brown paper bags containing wet paper towels. Leaves were examined for infection after 7 and 14 days. All isolates were pathogenic and produced non-sporulating lesions similar to those observed in natural infections. No symptoms were observed on noninoculated control plants. Disease incidence was low (<15%) until late June 1999 and July 2000. Inoculations in summer produced increasingly more infections, reaching incidences of 40 to 52% in September 1999 and 18 to 80% in August 2000. References: (1) J. E. Adaskaveg. 1994. Pages 5–7 in Proceedings of the 22nd Annual Almond Industry Conference. 1994. (2) J. Rotem. 1994. The genus Alternaria. Biology, Epidemiology, and Pathogenicity. APS Press, St. Paul, MN. (3) E. G. Simmons. Mycotaxon 70:325–369, 1999.

Anthracnose is the most destructive fungal disease on aerial parts of cashew trees. In this review, we addressed the current taxonomy of Colletotrichum species that infect cashew trees in Brazil, as well as their geographic distribution and epidemiological aspects of anthracnose. For a long time, cashew anthracnose was attributed exclusively to C. gloeosporioides, but recent studies revealed at least seven Colletotrichum species as responsible for this disease in cashew trees in Brazil. Among the identified species, C. siamense was the most frequent, occurring in all geographic regions on both cultivated and wild cashew trees. The optimal temperatures for mycelial growth and conidial germination of Colletotrichum spp. ranged between 25–30 C and 27–37 °C, respectively. The wide host range of Colletotrichum species associated to cashew plants constitutes a serious problem for anthracnose management, which is usually focused on chemical control. To a lesser or greater extent, Colletotrichum species were sensitive to azoxystrobin, difenoconazole and thiophanate-methyl. The correct identification of the Colletotrichum species responsible for anthracnose is essential to understand their interactions with the environmental factors that influence the epidemiology of the disease in different hosts, as well as for the improvement of disease control strategies and plant breeding programs aimed to find resistant crop varieties.

Daylily (Hemerocallis citrina Baroni) is a perennial herb whose flowers are commonly used in traditional Chinese cuisine. It is commercially cultivated in the Loess plateau of Gansu province, China. From July to October 2020, necrotic lesions were observed on the foliage of daylily plants in Huan County, Gansu, China, with an average disease incidence of 90%, and 52 to 86 disease index across four fields (approximate 6 hectares). Lesions were fusiform or nearly fusiform yellowish-brown spots of different sizes and a yellow irregular border. Older lesions were almost dark brown that often coalesced and expanded to cover the entire leaves. Thirty-four samples were collected from plants with typical foliar symptoms. Symptomatic tissues were excised from the margins of the lesions and sterilized with 75% ethanol for 20 s and 0.1% NaClO for 2 min, rinsed with sterilized water four times, dried on sterile paper towels, and cultured on Potato Dextrose Agar medium at 25°C for 7 days. A total of 34 fungal isolates with 100% isolation frequency were obtained and characterized. Colonies were white, becoming pale brown with age, reverse turned grayish black with age and irregular pale yellowish borders on the reverse side. Conidia (n=50) were hyaline, one-celled, subcylindrical with obtuse to slightly rounded ends, of 12-18.5×3.5-6 µm in size, (avg. 15.5×4.8 µm). The isolates were designated as K2010301 (51-54) and deposited in the Microbiological Culture Collection Center at College of Pastoral Agriculture Science and Technology, Lanzhou University (China). For fungal identification to species level, genomic DNA of a representative isolate (isolate MG) was extracted. Internal transcribed spacer (ITS) region, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), chitin synthase-1 (CHS-1) and beta-tubulin (TUB2) were amplified using V9G/ITS4, GDF1/GDR1, CHS-354R/CHS-79F, and T1/Bt-2b primer sets (Damm et al., 2012), respectively, and deposited in GenBank under accession numbers MW811458, MW836582, MW836581, and MW836584. BLASTn showed higher than 99% identity with Colletotrichum siamense (GenBank: KP703350 (ITS), MN884050 (GAPDH), MN894598 (CHS-1), and KX578815 (TUB2)). A Bayesian inference analysis of the four concatenated loci showed that isolate MG grouped in the C. siamense clade. Pathogenicity tests were performed by spraying a spore suspension (1×105 conidia/mL) of a 10-day-old culture of isolate “MG” onto 3 healthy and asymptomatic daylily plants. Three control plants were only sprayed with the same volume of sterile distilled water. The inoculated plants were covered with black plastic bags for 2 days to maintain high relative humidity. Anthracnose symptoms resembling those observed in the field developed after 7 days on all inoculated plants, while no symptoms were observed on the control plants. The fungus was reisolated and identified as C. siamense based on morphological features and DNA sequence analysis, fulfilling Koch’s postulates. It has been demonstrated that C. liliacearum (Zhuang, 2005), C. gloeosporioides, and C. spaethianum (Yang et al., 2012) are anthracnose pathogens of H. citrina. To our knowledge, this is the first report of C. siamense causing daylily anthracnose worldwide. This fungal pathogen represents a severe threat and has the potential to cause yield losses of daylily, so further studies should focus on epidemiology and effective management strategies of this disease.

In Australia, anthracnose on almond is caused by the ascomycete fungus Colletotrichum acutatum. Anthracnose has been confirmed throughout the major almond growing regions of Australia and significant economic losses have been reported. Fungicide trials carried out in Australia in 2000, prior to this project, yielded variable results that differed from those achieved in California. Two subpopulations of C. acutatum from almond have been reported in California, whereas one clonal population of Colletotrichum sp. from almond has been described in Israel, which differed from the subpopulations of C. acutatum in California. A collection of isolates of C. acutatum from almond in Australia was established. The isolates were characterised with respect to morphology, genetic variation and pathogenicity to detached plant tissues, and compared to representative isolates of Colletotrichum sp. from California and Israel. Plant material was cultured at regular intervals for isolation of C. acutatum to determine which tissues were likely to be the main sources of inoculum. The development of anthracnose on almond was monitored in the field for three successive growing seasons and relationships with weather data examined to elucidate the environmental conditions that are most conducive for disease. There was considerable variation among isolates of C. acutatum from almond in Australia in terms of morphological and cultural characteristics. However, three main morphotypes were evident, namely pink, orange and cream colony colour. In general, isolates of C. acutatum from Australia were more similar morphologically to the pink subpopulation of C. acutatum from California than to the grey subpopulation from California and the isolates of Colletotrichum sp. from Israel. Isolates of Colletotrichum sp. from almond in Australia were confirmed as C. acutatum by means of PCR with C. acutatum-specific primers. Subsequently, genetic variation was investigated using PCR with inter-simple sequence repeat primers, and the data were clustered using UPGMA. All isolates of C. acutatum from almond in Australia, except for one, shared 100% genetic similarity to one another, suggesting that the population of C. acutatum from almond was likely to be largely clonaL The isolates of C. acutatum from almond in Australia were genetically distinct from the isolates of the pink and grey subpopulation of C. acutatum from almond in California and from the Colletotrichum sp. from almond in Israel. Pathogenicity experiments on detached leaves and fruit revealed pathogenic variation among representative isolates of C. acutatum from almond in Australia, California and Israel, however, all isolates tested caused disease symptoms. The susceptibility of the main almond cultivars grown in Australia was examined by inoculating detached leaves and fruit with isolates of C. acutatum from almond in Australia, California and Colletotrichum sp. from Israel. The results were inconclusive, and further research is needed to develop a rapid and reliable screening method to assess cultivar susceptibility. The isolation of C. acutatum from almond tissues monthly for one year suggested that mummified fruit, peduncles and woody tissue were potentially significant sources of primary inoculum. These findings support the recommendation that the removal of mummified fruit and associated woody tissue may reduce inoculum potential and subsequent disease. Correlating disease incidence in the field with weather data showed that rainfall early in the growing season appeared to be important in the development of anthracnose. Infection of almond tissues occurred when fruit was young, and disease incidence did not increase beyond November in two out of the three years during which disease was monitored. Disease progress curves and relative area under the disease progress curve data showed significantly greater disease incidence on Price than Nonpareil, whereas the apparent rate of infection for Nonpareil and Price was similar for 2002 and 2003. On balance, these results suggested that there was little difference in susceptibility between Price and Nonpareil, but disease incidence may differ due to other factors, such as timing of fruit set, however, further investigation is needed to substantiate this. These results endorse the current recommendation that preventative fungicide sprays commence early in the growing season, however, sprays may not be necessary beyond November.
Thesis (Ph.D.) -- University of Adelaide, School of Agriculture and Wine, 2005