Alternaria is the name given to a group of fungal plant pathogens that are responsible for the disease Alternaria leaf spot. The disease Alternaria leaf spot is caused by the fungus Alternaria, and is found on over 200 different crops and plants. This disease usually occurs on leaves, but can also attack fruit, vegetables, and grains.

Leaf spot (Image source: Sac Valley Orchards

Conditions for Alternaria growth

Alternaria leaf spot affects crops of all kinds. It is particularly difficult to control because of its wide host range. This disease is most commonly associated with warm and humid conditions. Temperatures of 77-86°F (25-30°C) and high humidity, such as that found in greenhouses or rain gardens, favor the disease.

According to Saad and Hagedorn (1970), minimum, optimum and maximum pH for the growth and sporulation of Alternaria alternata were found to be 4.40, 6.50 and 7.60 respectively.

Kaur and Kumar (2019) reports a survey of results on the optimum growth conditions of various alternaria species. According to their report:

  • Optimum temperature: 77-83°F (25-28°C). Growth declines after 95°F (35°C) and halts at 113°F (45°C). Colder conditions below 10°C
  • Optimum pH: 6.5. (Minimum 4.40 and maximum 7.60)
  • Optimum humidity: 93% to 100%. At %74 relative humidity and below, there seems to be slower growth. In multiple studies, a relative humidity higher than %80 is considered to increase alternaria lesions.

Mechanism of action

Alternaria leaf spot can be a major disease in fruit crops. A typical disease incidence is 25-50 percent. In some cases, Alternaria leaf spot can cause crop loss or even death.

According to Meena and Samal (2019), Alternaria species secrete more than 70 mycotoxins and several species produce phytotoxins that have detrimental effects on most cell structures such as chloroplast, mitochondria, Golgi bodies, cell membrane, nucleus etc.

Examples of non-host-specific toxins produced by Alternaria species:

  • tentoxin (TEN)
  • Alternaric acid, alternariol (AOH)
  • alternariol 9-monomethyl ether (AME)
  • brefeldin A (dehydro-)
  • Alternuene (ALT)
  • Altertoxin-I, Altertoxin-II, Altertoxin-III
  • zinniol
  • tenuazonic acid (TeA)
  • curvularin
  • alterotoxin (ATX) I, II, III

Examples of host-specific toxins produced by Alternaria species:

  • AK-toxin
  • AF-toxin
  • ACT-toxin
  • AM-toxin
  • AAL-toxin
  • ACR-toxin
  • maculosin
  • destruxin A, B

These host-specific mycotoxins such as AK-toxin, AF-toxin, and AC-toxin have the devastating effects on plants causing cytotoxic, apoptotic cell death, DNA breakage, changing membrane permeability and interrupting plant physiology by mitochondrial oxidative phosphorylation.

Alternaria leaf spot appears as small, round, yellowish-green to gray-green spots on leaves. This disease is most often seen on leaves of the central part of the plant.

Symptoms

Alternaria leaf spot produces a wide range of symptoms. Some of the more obvious symptoms are a reduction of leaf area, necrotic spots on leaves, stunted growth, and defoliation. Alternaria leaf spot can affect many different plant parts.

In many cases, Alternaria leaf spot appears on leaves as a leaf spot. These spots appear as fairly large brown spots: about 0.5 to 0.75 inches (12–18 mm) in diameter. [1] Infections appear as brown, circular lesions that coalesce into large, irregular necrotic zones on leaves. Lesions often become black in the center where numerous spores are produced. Rapidly developing in June and July, defoliation can hit the trees by early summer when the disease is severe.

Alternaria leaf blight first infects mature leaves near the crown of the plant. Fruit can suffer from sunscald due to leaf loss.

Warm temperatures, dew and humidity are ideal conditions for Alternaria leaf spot development. This disease is consistently worse in areas of little or no air movement and where dew remains for many hours each day in late spring through summer. [2]

Management

  • Almond varieties including Carmel, Sonora, Monterey, Winters and Butte are among the most susceptible to Alternaria.
  • Watch for early signs of the disease between from April to June. If Alternaria present, late-spring treatments may begin about mid-April.
  • In orchards that were exposed to Alternaria at any point in time, treat in mid-to-late April and treat again 2-3 weeks later.

According to Almond Board of California [3], most effective antifungal management strategy in 2016 was to use these strictly in rotations (to avoid resistance):

  • Inspire
  • Fontelis + Aproach
  • Luna Experience
  • Merivon
  • Ph-D, Ph-D + U12
  • UC-2B

[1] University of California Agriculture and Natural Resources https://www2.ipm.ucanr.edu/agriculture/almond/Alternaria-Leaf-Spot/

[2] Disease Identification Guide for Almonds and Pistachios https://www.syngenta-us.com/crops/image/almonds_assets/almond-pistachio-disease-id-guide.pdf

[3] Pest Management and Pollination - Almond Board of California https://www.almonds.com/sites/default/files/research_update_pest_management_and_pollination.pdf