Fruit Russeting (Powdery Mildew-Like)

Russeting on almond hulls is reminiscent of rusty spot on peach fruit caused by P. leucotricha, but the typical powdery white growth is absent. No conidia (asexual spores) or chasmothecia (sexual fruiting structures) of powdery mildew fungi have been observed. Only thin hyphae, not characteristic of powdery mildew fungi, are present. An Acremonium species has consistently been observed sporulating on the fruit. Foliar and twig symptoms are absent.

The cause of fruit russeting of almond is unknown, but powdery mildew fungi (Podosphaera pannosa, P. tridactyla, and P. leucotricha) have been implicated. The absence of characteristic powdery mildew conidia and the presence of hyphae not typical for powdery mildew fungi provide evidence that different fungi are involved. More recently, an Acremonium species has been consistently observed growing and sporulating on, and has been isolated from, diseased fruit. Studies are currently being conducted to conclusively determine the pathogens involved.

Numerous fungicides with different modes of action are effective against the disease, including many that are effective against powdery mildew (e.g., demethylation inhibitor (DMI) and quinone outside inhibitor (QoI) fungicides) and others that are not (e.g., iprodione). The difficulty in working with powdery mildew fungi and Acremonium spp. has made absolute identification of the pathogen complicated.

The disease is only rarely reported in California and generally does not cause economic crop losses. Growth of the pathogen is favored by cool moist nights and warm days. Generally, fruit are susceptible only up to time of shell hardening, but later infections can occur on some varieties. Certain cultivars such as Mission, Padre, and Ruby are most susceptible.

The disease is rarely an economic problem on almonds in California. If management is needed, control practices should focus on protecting fruit from infections. Jacket-split (late petal fall) and mid-spring applications of fungicides are highly effective in managing the disease.

Some sulfur sprays are acceptable for use in an organically certified crop.

Generally, chemical control is not needed. If necessary, apply a fungicide from jacket split until mid-April.