Venturia Inaequalis (Cooke) Winter, Hedwigia 36: 81, 1897

Set No 41, published 1974

CMI Descriptions of VENTURIA INAEQUALIS Pathogenic Fungi and Bacteria No. 401

Venturia inaequalis (Cooke) Winter, Hedwigia 36: 81, 1897. Sphaerella inaequalis Cooke, 1866. Spilosticta inaequalis (Cooke) Petr., 1940. Endostigma inaequalis (Cooke) Syd., 1923. Sphaeria cinerascens Fuckel, 1863. Sphaerella cinerascens Fuckel, 1870. Conidial state: Spilocaea pomi Fr., 1825. Fusicladium pomi (Fr.) Lind, 1913. Helminthosporium pyrorum Lib. (pro parte), 1832. (For further synonymy see Barr, Canadian Journal of Botany 46: 808, 1968 and Hughes, Canadian Journal of Botany 31: 566–568, 1953.) Pseudothecia immersed, globose, amphigenous, scattered or grouped, with or without setae. Asci cylindrical, bitunicate, 8- spored, 60–70 × 7–12 µm. Ascospores monostichous or distichous, olivaceous brown, septate in the upper third, with upper ends tapering and lower ends rounded, 12–15 × 6–8 µm. Conidial state: Conidiophores arising from subcuticular or intraepidermal mycelium which forms radiating plates, simple cylindrical, pale to mid brown to olivaceous brown, sometimes swollen at the base, variable in length, up to 90 µm long, 5–6 µm wide. Stroma often formed as pseudoparenchyma. Conidia produced singly at the tip of the conidiophore and then successively by proliferation through scars of the detached conidia, resulting in characteristic and distinct annellations on the conidiophores; obpyriform to obclavate, pale to mid olivaceous brown, smooth, 0–1-septate, 12–30 µm long, 6–10 µm wide in the broadest part with a truncate base 4–5 µm wide.

HOSTS: Principally on apple (Malus pumila), and other species of Malus. Also recorded on Pyrus spp., Sorbus spp. Pyracantha, Cotoneaster integerrima, Crataegus oxyacantha, Viburnum, Sarcocephalus esculentus (36, 278) (Herb. IMI). DISEASE: Causes scab or black spot of apple, a common disease which can produce serious losses in both quantity and quality of fruit. The fungus can infect shoots, buds, blossoms, leaves and fruit. Symptoms appear initially as small, dull, pale spots particularly on the underside of leaves in spring. These enlarge as a mass of radiating subcuticular hyphae develops, forming dark circular lesions visible on both leaf surfaces; a slight puckering or blistering effect may also occur. Older lesions turn grey as the infected host tissue is killed. Scab on fruit is usually more conspicuous than on leaves, but varies according to resistance, pathogen virulence etc. Severe lesions may involve most of the fruit and become suberized and cracked, thus allowing entrance of secondam rotting organisms. Early infection of young fruit may cause shedding or distorted growth. On older fruit smaller secondary, lesions (pepper spot) often develop around a large primary scab. Infection of mature fruit may become visible during storage as sunken, black lesions. Lesions on young shoots appear as light brown blister-like swellings. GEOGRAPHICAL DISTRIBUTION: Worldwide wherever apples are grown (CMI Map 120, ed. 3, 1966). PHYSIOLOGIC SPECIALIZATION: Isolates of the fungus are highly variable in both morphological and physiological characters (13, 383) and variation in different geographic populations occurs (39, 28). Work with biochemical mutants has shown that virulence may be determined by nutritional independence (37, 411). Several physiologic races have been determined by their differential pathogenicity to a range of Malus species and varieties (35, 829; 45, 3366; 48, 518). Formae speciales have been proposed for some strains showing special pathogenicity to different genera in cross inoculation tests (36, 278) as follows: Venturia inaequalis f.sp. mali on apple, f.sp. aucupariae on Sorbus spp., f.sp. cotoneasteris on Cotoneaster integerrima, f.sp. crataegi on Crataegus oxyacantha (which is erroneous and refers to V. crataegi). TRANSMISSION: The fungus overwinters primarily on leaf litter where the saprophytic sexual (perithecial) stage occurs. In wet spring weather the resulting perithecia release masses of airborne ascospores which infect the susceptible young foliage (42, 204). Dormant overwintering lesions on shoots and bud scales may also occur; these produce conidia which can infect young spring growth. Primary (spring) lesions produce conidia which cause secondary infection of foliage, fruit and shoots during wet summer weather. Conidia are dispersed chiefly by rain splash but may be present in the air near heavily infected trees during dry conditions (40, 757). NOTES: The fungus can be grown on artificial media such as 3% malt agar but perithecia are produced only when N is low and production is then influenced by the N source (50, 3490). Production of perithecia on leaf litter during the perfect, winter, stage of the life cycle requires the presence of compatible strains of the fungus (V. inaequalis is heterothallic) and certain environmental conditions. On apple leaf disks, perithecia were produced only after 4–5 months at 4–8°C (41, 468, 723). Asci mature continuously over a fairly long period, but ascosporoa are released only during a few hours after thorough wetting. Far red light also stimulates ascospore release (48, 2455). Ascospores may be produced from infected leaf litter over a period of 2–3 months, the amount in orchard air is usually greatest during the day and is influenced by the maturation rate relative to the frequency of release, while the total dose available depends upon the amount of previous leaf infection and leaf litter survval during the winter (42, 204). Germination of ascospores requires certain conditions of temperature and wetness duration (Mills & Laplante, Extension Bulletin. Cornell University Agricultural Experiment Station 711, 1954) and the occurrence of these conditions is used to predict disease outbreaks which occur about one month later (40, 694). In England, total April rainfall usually determines scab severity. Many countries have scab warning services which detect potential infection periods from meteorological data; these are used to facilitate control by curative sprays. Conditions for conidial germination and infection are similar to those required by ascospores, but secondary infections during cool wet summer weather are often limited, particularly if primary spring infection has been adequately controlled. Many species and varieties of Malus show various degrees of resistance to scab (42, 691; 46, 2059) which seem to be controlled by a series of allelic genes, but most widely grown commercial varieties are susceptible. Some, e.g. M. atrosanguinea (41, 396), show a hypersensitive reaction to infection. This resistance is associated with the release of phloridsin by the penetrated tissues and its subsequent conversion to oligomeric quinones (48, 844, 1813). Resistance has also been correlated with vigour (43, 1072), low organic acid and high catalase and peroxidase activities (51, 456) and pubescence (51, 3436). Root stocks may also influence scion susceptibility to leaf scab (43, 1692). Control is achieved by the application of fungicides by either high volume (200 gal/acre) hydraulic sprayers or lower volume (100 gal/acre or less) air blast atomisers to achieve good coverage of foliage. There are three main spraying schedules related to the life cycle of the fungus: (i) postharvest spraying to prevent winter development of perithecia and spring production of ascospores; (ii) a protective schedule applied at about 14-day intemals from bud burst to fruitlet stage, and (iii) post infection or curative spraying applied within 48 h of an infection period to eradicate new infections. Mixtures of these schedules are often used (47, 250). Copper fungicides, dithiocarbamates and lime-sulphur have been used as protectants, but some of these cause fruit russetting on certain varieties. Captan and dodine, which has some eradicant action, are widely used as protectants and phenyl mercury, salts as postharvest and curative sprays. O-coumarin esters have shown promise (44, 3090). Applications of 5% urea spray just before leaf fall greatly reduce subsequent penthecial development as breakdown of leaf litter is hastened and an antagonistic microflora develops (44, 1609; 48, 517). Recently systemic fungicides have been shown to be very effective as protectants (51, 2641) and postharvest sprays (51, 4130), e.g. benomyl, triarimol, thiophanate. Resistance to fungicides has also been noted (46, 2461; 49, 1701).

LITERATURE: Wallace, Bulletin Cornell University Agricultural Experiment Station 335: 543–624, 1913; Keitt & Jones, Bulletin, Wisconsin Agricultural Experiment Station 77, 1926; Anderson, Diseases of fruit crops, 1956; Hirst & Stedman, Annals of Applied Biology 49: 290–305, 1961; 50: 525–567, 1962 (epidemiology); Raa, Natural resistance of apple plants to Venturia inaequalis, Univ. Utrecht, 1968 (thesis). A. Sivanesan & J.M. Waller