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Alternaria alternata

Alternaria alternata causes leaf spot disease, rots and blights on a wide range of host plants. The fungus belongs to the opportunistic pathogens.

Symptoms

Alternaria leaf spot appears as fairly large brown spots on leaves, about 12-18 mm size in diameter. The fungus produces spores, which are seen as black spots. Leaf spot develops most rapidly in June and July, and trees can be almost completely defoliated by early summer when the disease is severe. The disease appears to be most severe where dews form, humidity is high, and air is stagnant.

Alternaria leaf spot can occur on almond trees grown anywhere in the Central Valley, but rarely is it severe enough in the northern San Joaquin Valley to require treatment. It has been most serious on trees in the southern San Joaquin Valley and in the northern Sacramento Valley.

Disease Cycle

The asexual spores of the fungus are thick-walled, multicellular, and pigmented and are able to olerate adverse conditions like dry weather. Spores are produced on leaves and in lower numbers on fruits and twigs remaining on the tree as well as on leaf litter. When there is no susceptible tissue available, such as over the winter, the fungus survives on mature leaves, twigs and fruit. Spore production is greatest when relative humidity is above 85%. Spores are air-borne and release into the air is triggered by rainfall or by a sharp change in relative humidity. Once the spores are released, they are moved by wind to susceptible tissue where they are able to infect. When temperatures are favorable 20-29°C, the length of the wetting period required for infection is about 8-10 hours. When temperatures drop below 17°C or rise above 32°C  and , the fungus requires extended leaf wetness durations (>24 hrs) to cause significant infections.

On highly susceptible cultivars, as little as 6 hours of leaf wetness can result in infections. Most of the infections probably follow a rainfall event, but dew can be sufficient to bring about infection. For example, in Israel and Spain, little rain occurs after petal fall, but they can still have significant infection as the result of heavy dews.

Disease model

Disease model originated from http://www.ipm.ucdavis.edu/PMG/r3101611.html#REFERENCE

The model calculates a disease severity value (DSV) per day (similar model as the TomCast in Tomato).

Disease severity values depend on the average temperature during leaf wetness periodes during a day. Application of fungicides is recommended if an accumulated index value of 10 DSV or higher is reached over a 7 day periode.

Mean temperatures
(C) during wetness
Leaf wetness duration (hours)
15-17 0-6 7-15 16-20 21 ---
17.1-20 0-3 4-8 9-15 16-22 23+
20.1-25 0-2 3-5 6-12 13-20 21+
25.1-29 0-3 4-8 9-15 16-20 23+
DVS 0 1 2 3 4


In some orchards, Alternaria sp. resistance to quinone outside inhibitor (QoI) fungicides (also known as strobilurins) and succinate dehydrogenase inhibitor (SDHI) fungicides have been documented; do not use FRAC mode of action Group number 7 or 11 fungicides in these orchards.

Management

The disease occurs first and is most severe on exposed leaves. Trees trained to an open and spreading canopy usually have more severe Alternaria leaf spot. Trees planted with rows in an east-west direction also have more severe disease than do orchard with rows planted north-south. Varieties that are most susceptible include Carmel, Sonora, Monterey, Winters, and Butte. Monitor for signs of the disease in April through June. If monitoring indicates the presence of Alternaria, begin late-spring treatments about mid-April. In orchards with a history of the disease, treat in mid- to late April and 2 to 3 weeks later.

Source:
Dewdney M. M. (2013). Alternaria Brown Spot- Document PP152, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date July 2001. Reviewed May 2008. Revised June 2013. Visit the EDIS website at http://edis.ifas.ufl.edu

Adaskaveg J. E., Gubler W. D., Duncan R., Stapleton J. J., Holtz B. A. (2012) UC IPM Pest Management Guidelines: Almond; UC ANR Publication 3431. page: http://www.ipm.ucdavis.edu/PMG/r3101611.html#REFERENCE