Mycosphaerella gibsonii
(needle blight of pine)

Mycosphaerella gibsonii is a fungal pathogen causing needle blight, primarily in Pinus species. It causes lesions on needles, first affecting lower needles and then spreading to the upper crown. The disease eventually causes needle necrosis and needle cast, leading to defoliation, stunted growth and host plant death; it is a major obstacle to the production of pine seedlings. M. gibsonii occurs in the tropics and subtropics of South and Central America, the Caribbean, sub-saharan Africa, India, South East Asia and East Asia; the native range is uncertain. Although natural dispersal by wind and water occur locally, international spread is largely due to movement of infected nursery stock. Phytosanitary control measures such as avoiding the planting of infected plants, removal and destruction of all infected pines in nurseries and cleaning between annual production cycles in nurseries can help to reduce the spread of the pathogen. It is listed as an A1 quarantine pest in the EPPO region, and is considered of quarantine significance in South America. 

The fungus is much more widely distributed in tropical and subtropical regions than previously supposed (Ivory, 1994). It occurs across the tropics and subtropics of South and Central America, the Caribbean, sub-saharan Africa, India, South East Asia and East Asia, where it may affect native Pinus species (Quaedvlieg et al., 2012). Records from native Pinus wallichiana and P. roxburghii in Nepal suggest that this fungus could be native to the Himalayas.

See also CABI/EPPO (1998, No. 216).

In the EPPO and South Cone Plant Protection Committee (COSAVE) regions it is potentially dangerous to Pinus spp. wherever they are grown. Nursery trade increases the risk of spread of M. gibsonii. It is listed as an A1 quarantine pest by EPPO (OEPP/EPPO, 1980). It is also of quarantine significance to the Andean Community (CAN) and COSAVE (Brazil, Ministério da Agricultura e do Abastecimento, 1996; COSAVE, 2018). 

M. gibsonii affects many species of Pinus. Additionally, it has recently been reported on Abies nobilis [Abies procera] in Japan (Farr and Rossman, 2018). It is particularly associated with old seedlings in tree nurseries, but is also quite common on older trees of highly susceptible species such as P. canariensis, P. radiata and P. roxburghii. Other susceptible species include P. halepensis, P. pinaster and P. sylvestris (EPPO, 2018). It also occurs on senescing foliage and leaf litter of many species at a much lower incidence level (Ivory, 1994).

During field surveys carried out by Ivory (1994) in Africa, Asia, Central America and Oceania, the fungus was found on Pinus ayacahuite, P. canariensis, P. caribaea, P. clausa, P. densiflora, P. elliottii, P. greggii, P. halepensis, P. kesiya, P. luchuensis, P. massoniana, P. merkusii, P. muricata, P. oocarpa, P. patula, P. pinaster, P. pseudostrobus, P. radiata, P. roxburghii, P. rudis [P. hartwegii], P. sylvestris, P. taeda, P. thunbergii and P. wallichiana. Other reported host species include P. cembra, P. contorta, P. echinata, P. flexilis, P. jeffreyi, P. lambertiana, P. mugo, P. nigra, P. parviflora, P. pinea, P. ponderosa, P. resinosa, P. rigida, P. strobus, P. taiwanensis, P. tabuliformis (Braun et al., 2013), P. maximinoi, P. morrisonicola, P. tecunumanii (Chen, 1965), P. armandii, Tsuga canadensis and Abies nobilis (Farr and Rossman, 2018).

It must also be noted that the following conifer species were found to be susceptible to the pathogen under artificial inoculation experiments: Abies veitchii, A. sachalinensis, Cedrus deodara, Picea glehnii, P. jezoensis, Pseudotsuga menziesii (Suto, 1979; Diekmann et al., 2002) and Larix kaempferi (Kobayashi et al., 1979; Suto, 1979; Diekmann et al., 2002).

Physiology and Phenology

Isolates of M. gibsonii from Asia differ distinctly from African and Jamaican isolates. A third type, which has many similarities with cultures of M. dearnessii, was found on Pinus caribaea in the Philippines (Ivory, 1994). Due to differences in conidial morphology, there are probably three ecotypes: Asia, Africa-Central America and Philippines (Ivory, 1994). M. gibsonii has been found in one or more of its three spore forms (teleomorph, conidia or spermatia) on old seedlings and small trees associated with a more or less severe needle blight of the lower crown, on senescing foliage of older trees and on hanging and dry surface litter (Ivory, 1994). Old foliage is usually infected first, but in severe cases all foliage can be infected (Crous et al., 1990). However, Singh et al. (1988) observed that primary needles appeared to be more susceptible to attack by the pathogen than mature needles.

Conidia germinate between 10 and 35°C (25°C being optimal) and the incubation period varies with environment, but is thought to be 4-6 weeks (Gibson, 1987). According to Suto (1979), heavily infested plants may have a shorter incubation period than those that are moderately infected. In Japan, Ivory (1972) indicated that the disease requires 2-3 days of moist humid conditions for dispersal and infection. Ivory (1987) indicated that under ideal conditions, 3-7 days is sufficient for the production of spores, their dispersal and needle infection by the pathogen. Following infection, stromata form in the stomatal cavities, and bear dense conidiophores. M. gibsonii overwinters as mycelial masses or immature stromata in the tissues of diseased needles (Ivory and Wingfield, 1986). M. gibsonii may also overwinter as latent infections in asymptomatic needles in instances where the needles were infected later in the year (Ivory and Wingfield, 1986; Diekmann et al., 2002; Sullivan, 2010). Symptoms associated with latent infections are often observed the following spring (Ivory and Wingfield, 1986). See also Chupp (1953), Ito (1972), Mulder and Gibson (1972) and Deighton (1987). 

M. gibsonii can be grown in pure culture quite easily, but spores other than spermatia are rarely produced (Ivory, 1994). The spermatial anamorph is known as Asteromella sp.

Environmental Requirements

There is some evidence of edaphic factors (soil pH, organic matter and nutrient content of soil) affecting the incidence of needle blight (Cruz et al., 1984). However, Singh et al. (1988) suggested that low incidence of the disease during winter and dry summer months may be due to low and high temperature respectively. Warm, damp weather provides ideal conditions for conidia production, germination and infection while dry weather often inhibits conidia production (Ivory and Wingfield, 1986). Although conidia are not produced during dry weather, the fungal pathogen may remain dormant in host tissues (Gibson, 1979). It is possible that inocula can survive under unfavourable conditions as mycelium in plant and plant residues (Gibson, 1987). Conidia can remain viable for about one month, but under moist conditions will germinate on needle surfaces within 24-40 hours and penetrate via stomata within a further 2-3 days (Ivory, 1987).

The disease may be confused with dothistroma blight (Mycosphaerella pini), but the pathogen may be distinguished by examination of the conidia (Suto, 1971). There is never a reddish tint to the necrotic needle tissues as may occur with other infections.

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control and Sanitary Methods

Gibson (1987) suggests that losses from the disease may be kept to a minimum by providing unfavourable conditions for spread and infection of conidiae of the pathogen. Some measures for control of M. gibsonii were listed by Ivory (1987). General nursery hygiene measures to minimize the risk of infection are listed below:

• Removal and destruction of all infected pines in and around the nursery
• Cleaning out the nursery between annual production cycles
• Physical separation of young seedlings from older plants where the nursery cycle is >12 months
• Banning transfers of plants between nurseries
• Taking care with mycorrhizal-soil introductions

Additional measures provided by Gibson (1987) for nurseries include:

• Setting up nurseries away from any plantation with diseased trees
• Proper weeding of nursery beds, pathways and nursery surroundings
• Production of robust seedlings by avoiding excessive shading, and hardening of the seedlings as soon as possible before the onset of the rainy season
• Use of temporary rather than permanent nursery sites (rotating nursery sites)
• Wide spacing of nursery plants (e.g. 15 cm)
• Minimizing needle wetting either by protecting against rain if possible or by avoiding overhead irrigation
• Avoiding the planting of infected plants
• Maintaining a proper weeding regime and pruning the lower branches to avoid direct contact with weeds

Singh et al. (1988) observed that in their experimental set-up, in nurseries where infected needles are present in the needle mulch or in mycorrhizal soil, infection appeared on needles within one month of pricking (transplanting). They recommended that:

• Existing nurseries where the pathogen is established are to be avoided in the set up of new nursery sites
• Needles or pine litter collected from infected plantations should not be used as mulch and should be collected and destroyed
• Seeds of exotic pines, when exported, should be completely freed from all seed debris before sowing in nurseries
• Planting schedules should be arranged outside of rainy months

Chemical Control

In nurseries, control can be obtained by using maneb (or mancozeb) or copper-based fungicides applied to the seedlings from the current year and 1-year-old seedlings at 2-weekly intervals during the growing season (Reddy and Pandey, 1973). It is important that all diseased seedlings are removed and burned early in the season when infection occurs. Apply foliar sprays of fungicides at 2-4 week intervals during conditions favourable for the spread of the fungus. Recommended fungicides are benomyl, chlorothalonil, mancozeb, zineb, tridemorph and thiophanate, applied with effective wetting agents (Singh et al., 1983; Ivory, 1987). Copper fungicides have been reported to be effective only in Japanese nurseries (Ivory, 1987).

Host Resistance

Resistant Pinus spp. may have a small portion of their foliage affected, but suffer little growth reduction and no mortality. Resistance has been reported on P. clausa, P. elliotii, P. kesiya, P. rigida and P. patula (Gibson, 1987; EPPO, 2018). Ivory (1987) suggests replacing highly susceptible species with a more resistant, but equally productive species, such as P. elliottii in place of P. massoniana (Ivory, 1987). 

22/10/18 Updated by:

Annika Minott, Caribbean Agricultural Research and Development Institute (CARDI), Cayman Islands