Epidemiology and Control of Citrus Greasy Spot in Different Citrus-Growing Areas in Florida

Analysis of the Fungal Diversity in Citrus Leaves with Greasy Spot Disease Symptoms

Citrus greasy spot (CGS) is a disease of citrus with worldwide distribution and recent surveys have revealed a high level of incidence and severity of symptoms of the disease in Sicily, southern Italy. Although Mycosphaerel la citri (anamorph Zasmidium citri-griseum) and other related species are generally considered as causal agents, the etiology of CGS is still unclear. Here, we report the use of an amplicon metagenomic approach to investigate the fungal communities on citrus leaves symptomatic or asymptomatic for CGS from an orchard in Sicily showing typical CGS symptoms. A total of 35,537 high-quality chimeric free reads were obtained and assigned to 176 operational taxonomic units (OTUs), clustered at 99 % similarity threshold. Data revealed a dominating presence of the phylum Ascomycota (92.6 %) over other fungal phyla. No significant difference was observed between symptomatic and asymptomatic leaves according to both alpha and beta diversity analyses. The family Mycosphaerellaceae was the most abundant and was represented by the genera Ramularia, Mycosphaerella, and Septoria with 44.8, 2.4, and 1.7 % of the total detected sequences, respectively. However, none of the species currently reported as causal agents of CGS was detected in the present study. The most abundant sequence type (ST) was associated to Ramularia brunnea, a species originally described to cause leaf spot in a perennial herbaceous plant of the family Asteraceae. Results exclude that CGS symptoms observed in Sicily are caused by Z. citri-griseum and, moreover, they indicate that a considerable part of the fungal diversity in citrus leaves is still unknown.

Relationship of the Severity of Citrus Greasy Spot, Caused by Mycosphaerella citri, to Ascospore Dose, Epiphytic Growth, Leaf Age, and Fungicide Timing

Greasy spot, caused by Mycosphaerella citri, produces leaf and fruit lesions and defoliates trees, resulting in reduced yields and fruit size. Techniques now available allow production of large numbers of ascospores and the quantification of epiphytic growth. The effects of ascospore dose, leaf age, and the timing of fenbuconazole sprays on epiphytic growth and disease severity was determined primarily on rough lemon seedlings in the greenhouse. Inoculation of leaves with 10⁴ ascospores/ml resulted in rapid development of epiphytic growth and symptoms. At lower doses, epiphytic growth and symptoms developed more slowly and were less severe. There was a linear relationship between log₁₀ of the ascospore dose and ratings of epiphytic growth and symptoms, and a linear relationship between the amount of epiphytic growth and symptom severity in greenhouse tests. On grapefruit trees treated with different fungicides in six field experiments, there also was a significant linear relationship between epiphytic growth of M. citri measured in August and symptom severity rated in February to March of the following year, but coefficients of determination were much lower than in greenhouse experiments. Leaf age from 10 to 60 days did not affect susceptibility of leaves to M. citri. Fenbuconazole applied up to 50 days prior to inoculation still reduced epiphytic growth and greasy spot severity under greenhouse conditions, but the postinoculation treatments were effective for only 30 days.

Ascospore Deposition and Epiphytic Growth in Relation to Fungicide Timing for Control of Greasy Spot Rind Blotch Caused by Mycosphaerella citri

Greasy spot rind blotch is a serious problem in Florida for the production of grapefruit (Citrus paradisi) for the fresh market. In the 1970s to the early 1980s, the disease was described in detail and the cause was determined to be Mycosphaerella citri, the same species responsible for greasy spot of foliage. The most appropriate timing for fungicide sprays was determined at that time, but peak ascospore release has changed in recent years. In the present study, the relationship of ascospore deposition and fungal growth on fruit was determined in order to more accurately time fungicide applications. Infection of fruit appears to occur similarly to that of leaves: by deposition of ascospores and germination to produce epiphytic growth followed by penetration of the fungus through stomata. Ascospore deposition occurred mostly in May and June, but epiphytic growth began only after the onset of the summer rainy season in June in 2002 and 2003. Ascospore deposition was lower in 2002 than in 2003, but development of epiphytic growth was similar in both years. Timing of fenbuconazole sprays was evaluated in the 2001, 2002, and 2003 seasons. Of the single-spray applications, those in July were the most effective, sprays in June and August were moderately effective, and those made in May or September were ineffective. Two- and three-spray programs from June through August were usually more effective than single sprays, and four monthly sprays from May to August were needed for a high level of control. Fungicide applications are needed about every 3 to 4 weeks after the beginning of the rainy season in June through August for a high level of control of rind blotch.

Mating and Pseudothecial Development in Mycosphaerella citri, the Cause of Citrus Greasy Spot

ABSTRACT Greasy spot, caused by Mycosphaerella citri, is a serious disease of citrus in the Caribbean basin. M. citri is a loculoascomycete and produces pseudothecia in decomposing leaves after intermittent wetting and drying. A new in vitro mating technique was developed for production of pseudothecia on sterilized leaf disks in petri dishes. Of the single-ascospore cultures that were recovered from individual asci, four were one mating type and four were a second mating type (tentatively designated mat+ and mat-), indicating that M. citri probably is heterothallic and bipolar like most other loculoascomycetes. Most populations of ascospores recovered from individual leaves or from leaves from groves of different citrus species and various locations had a 1:1 ratio of mating types consistent with random mating. Cytological studies demonstrated that the ontogeny of pseudothecial development was similar to other loculoascomycetes. The formation of mature pseudothecia required 30 to 45 cycles of wetting and drying of infected, dead leaves which required approximately 60 to 90 days. The in vitro system for pseudothecial production and the knowledge of the mating system in M. citri will facilitate genetic studies of this important pathogen.

Environmental Factors Affecting the Release and Dispersal of Ascospores of Mycosphaerella citri

ABSTRACT Greasy spot, caused by Mycosphaerella citri, produces a leaf spot disease affecting all citrus species in Florida and the Caribbean Basin. M. citri produces pseudothecia and ascospores, which are considered the principal source of inoculum, in decomposing leaves on the grove floor. In studies using a computer-controlled environmental chamber, a single rain event triggered release of most mature ascospores beginning 30 to 60 min after the rain event. Additional rain events did not bring about further release. High relative humidity without rain triggered release of low numbers of ascospores, but vibration and red/infrared irradiation had little or no effect on ascospore release. After three to four cycles of wetting and drying of leaves, all pseudothecia had matured and released their ascospores. In the field, ascospores were detectable starting about 2 h after the beginning of a rain or irrigation and most ascospores were released within 16 h. Ascospore release was greatest following rain events and somewhat less following irrigations, and low numbers of ascospores were detectable on days without precipitation. Ascospore numbers declined linearly with horizontal distance from the source and as a function of the logarithm of ascospore numbers with vertical distance. Low numbers of ascospores were detected 7.5 m above the ground and 90 m downwind from the grove. Ascospore release can be advanced by irrigating frequently during dry, nonconducive conditions to stimulate ascospore release when environmental conditions are unfavorable for infection, but the eventual effects on disease severity are uncertain.

Effect of Urea, CaCO3, and Dolomite on Pseudothecial Development and Ascospore Production of Mycosphaerella citri

Citrus greasy spot, caused by Mycosphaerella citri, produces lesions on leaves, followed by premature defoliation, and rind blotch on fruit. Ascospores produced in leaf litter represent the major source of inoculum. The effect of treatment of leaf litter with urea, CaCO₃, or dolomite on the development of pseudothecia and ascospore production was evaluated. In laboratory experiments, one urea application reduced production of pseudothecia and ascospores by up to 90%, but did not affect time of production of pseudothecia or ascospores or rate of leaf decomposition. Two applications of urea delayed leaf decomposition. As the rates of CaCO₃ or dolomite were increased, pseudothecial incidence, density, time to ascospore production, and total numbers of ascospores decreased and the rate of leaf compostion increased. Immature pseudothecia on leaves treated with urea or CaCO₃ degenerated and produced fewer ascospores per pseudothecium. The results observed in microplot studies in the field were similar to those observed in laboratory experiments. The number of days to pseudothecia and ascospore production and the pseudothecial incidence and density were negatively related to the rate of CaCO₃ or dolomite applied. Application of CaCO₃ dolomite, or urea to leaf litter can reduce inoculum and be useful in an integrated program of greasy spot management.

Relationship of Epiphytic Growth of Mycosphaerella citri to Greasy Spot Development on Citrus and to Disease Control with Fenbuconazole

Greasy spot, caused by Mycosphaerella citri, produces foliar lesions and severe defoliation of citrus trees. Ascospore production and deposition by M. citri, development of epiphytic growth, and symptoms were monitored on grapefruit trees in the field and on rough lemon trap plants for 2 years. Ascospore production and deposition peaked in April to May both years. However, epiphytic mycelium did not develop extensively until the summer rainy season was well underway in July. Some epiphytic growth and symptoms formed on trap plants placed in the grove for 2-week periods throughout the year. In the summer, epiphytic growth was apparent 15 days after the exposure period, and symptoms appeared about 60 days after exposure, but development was much slower in cooler and drier months. One or two fenbuconazole applications before the development of epiphytic mycelium in July completely controlled greasy spot on spring growth leaves for 12 to 18 months. Applications in July or August were less effective. Epiphytic mycelium developed more rapidly on summer growth; therefore, fungicide applications need to be timed more precisely.

Environmental Factors Affecting Pseudothecial Development and Ascospore Production of Mycosphaerella citri, the Cause of Citrus Greasy Spot

ABSTRACT Mycosphaerella citri, the cause of citrus greasy spot, produces pseudothecia and ascospores in decomposing leaf litter on the grove floor. In laboratory studies, the effect of wetting and drying and temperature on the formation, maturation, and production of pseudothecia and ascospores was evaluated on mature, detached grapefruit leaves. Production of pseudothecia was most rapid when leaves were soaked five times per week for 2 h per day, but pseudothecial density and total ascospore production were greatest when leaves were soaked three times per week for 2 h per day. In duration of wetting studies, 3 h per day, 3 days per week brought about the most rapid production, but 10 to 30 min per day resulted in production of the most pseudothecia and ascospores. Pseudothecia and ascospore production were greatest at 28 degrees C and declined rapidly at lower and higher temperatures. Maturation of pseudothecia was slow at 20 and 24 degrees C, but production was high at 24 degrees C; at 32 degrees C, pseudothecia matured rapidly, but degenerated quickly. No mature pseudothecia were produced on leaves maintained continuously under wet conditions. In field studies, leaves were placed on the grove floor monthly from April 2000 to September 2001. Pseudothecia production was rapid during the summer rainy season from June to September. Pseudothecia produced on leaves placed in the grove from October to May developed and matured more slowly but were produced in much larger numbers than in summer. The number of days to first pseudothecial initials, 50% maturation, first discharge of ascospores, leaf decomposition, as well as pseudothecial density and incidence, were negatively related to average temperature. Total ascospore production was unrelated to temperature.