Day: May 12, 2023

Spinach Crown Mites in Spinach

Emily Zobel on

Spinach Crown Mites in Spinach
by Jerry Brust, UME

Spinach crown mites Rhizoglyphus sp. feed within the folds of new leaves in the crown of spinach plants. This feeding causes the new leaves to become deformed as they grow (figs.1 and 2). Crown mite adults are extremely small bulbous nearly transparent mites that also may have a yellow-beige body color with reddish-brown legs (fig 3). A good characteristic to look for to identify these mites is the sparse long hairs mostly found on the back end of the mite (fig. 3). Crown mite eggs are spherical and clear and laid on the creased leaf surfaces in the crown area. Some reports state that crown mites can act as vectors for plant pathogens such as Pythium and Rhizoctonia, but this is not definitive.

Feeding by spinach crown mites can cause misshapen and ragged leaves with necrotic margins as they expand and crown leaves are distorted and wrinkled in appearance.
Figs. 1 & 2.) Crown leaves fed on by spinach crown mites are misshapen and ragged with necrotic margins as they expand and in the field the crown leaves are distorted and wrinkled in appearance. Photos by G. Brust, UMD.

The spinach crown mite is most damaging in soils high in organic matter and under cool moist conditions (weather conditions we have had this past week). Because these mites can consume organic matter they can survive in soils after the crop has been removed. This is one reason they are difficult to control as they can survive for fairly long periods of time with no crop being present. The other reason they are difficult to ‘control’ is we do not realize they are causing the problem until it is too late.

Spinach crown mite adult with sparse long hairs over its body.
Fig 3.) Spinach crown mite adult with sparse long hairs over its body. Photo by G. Brust, UMD.

Most control recommendations include sanitation and crop rotations as being important as are fallow periods. Pyrethroids are a possible chemical control as is Neem; any chemical control has to get down into the crown of the plant to have any chance of working. There has been little research conducted on the most efficacious material for these mites. Mostly what is needed are warm sunny days where spinach can grow well and the environment is not so conducive to the mites.

Russet on Apples: Current Understanding and Management

Emily Zobel on

Russet on Apples: Current Understanding and Management

By John Skae, Candidate for B.S. in Physiology and Neurobiology, and  Macarena Farcuh, Ph.D. Assistant Professor and Extension Specialist,  UMD

What is russet on apples?

Russet on apples is a disorder of the skin that results in discoloration and changes to the expected smooth texture of the skin of apples. Russet appears as a spectrum from mild brown weblike patterns to severe rough changes on the surface of apples and many variations in between (Fig. 1). Russeting is only skin deep and thus will not affect the flesh of the fruit. It can occur due to naturally-occurring weather conditions, particularly humid and wet weather.

Fuji apple expressing russeting.
Fig 1.) Fuji apple expressing russeting. Source: John Skae, University of Marylan

Russeting can ultimately reduce the market value of apples, decreasing grower profitability. According to the US Department of Agriculture, the presence of russet disqualifies apples from the US Extra Fancy, US Fancy grades if smooth, solid russeting is more than 10%, while excessive russeting or smooth net-like russeting exceeding 25% excludes fruit from the US No. 1 grade categories.

Apples can begin to russet within the first 30-40 days of development, starting at petal fall. The earlier the tissues are damaged, the more dramatic the damage will be. But it is important to mention that russet can also be developed later during the growing season. Russeting occurs because cracks begin to develop underneath the cuticle of the apple. The damage in the epidermal cells underneath the cuticle turns brown. The cells are then pushed upwards towards the skin because new cork cells are growing underneath the affected epidermal cells. Once the damaged cells reach the surface of the apple, they form phellogen, a wound-sealing tissue created as a result of the russeting reaching the surface. Although russeting affects the cosmetic appearance of apples, it does not harm fruit flesh taste.

What factors cause or contribute to apple russeting?

Some cultivars produced by selective breeding are more prone to russeting than others as these can alter the genetic makeup of the apple to express russet. For example, Golden Delicious apples are highly susceptible to russet, while Red Delicious apples rarely express the russeting disorder.

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