A member of the family Amaranthaceae, Spinach is an annual plant grown mostly in temperate regions.
It has broad green leaves and is a common edible vegetable enjoyed in Asian countries. The plant is hardly consumed raw and is instead steamed to reduce the bitter oxalate taste.
The leaves are usually triangular, with broader leaves at the bottom and the plant typically achieves a height of one foot (30 cm). Though an easy plant to cultivate and grow, Spinach plants can often develop white spots.
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What Causes White Spots On Spinach?
White spots also referred to as white rust, are a fungal disease primarily caused by cold and moist weather. It may result due to incessant rain, continuous cropping, or over-irrigation of soil. Cool growing conditions and poor plant hygiene support the development and spread of the fungal pathogen. Preventative measures include suitable irrigation methods, crop rotation, and hygiene control.
Three Main Causes of White Spots on Spinach Leaves
1. Wet and Cold Growing Conditions
White spots are the symptoms of the fungal disease, White rust, caused by Albugo occidentalis. It is an obligate fungal parasite that fulfills its nutritional requirements by primarily attacking Spinach plants.
White rust disease is typically common throughout the United States, especially in Spinach fields east of the Rocky Mountains.
Albugo occidentalis is an oomycete that reproduces through zoospores. Zoospores are asexual spores that use their flagellum for locomotion. The zoospores require a temperature range of 57 degrees Fahrenheit to 64 degrees Fahrenheit (13-17 degrees Celsius) and a sufficiently moist environment to cause an infection.
Therefore, infection occurrences are more common during the spring and fall seasons. In these months, the temperature falls relatively low, and humidity levels increase.
Chilly mornings encourage the formation of dew, which subsequently enable A.occidentalis to reproduce asexually and form sporangia.
Sporangia develop into motile zoospores that thrive in the water. During the early stages of the infection, chlorosis occurs, and Spinach leaves turn yellow.
Chlorosis refers to the plant’s inability to form sufficient chlorophyll, which inhibits carbohydrate production through photosynthesis.
If these growing conditions persist, the zoospores germinate and cause small oval-shaped, white pustules to form on the leaves’ abaxial surface. Over time, the infection spreads to the entire plant, hindering growth and development.
Eventually, infected leaves turn brown and die.
2. Continuous Cropping
Continuous cropping is the practice of replanting new crops (of similar species) in the same soil mixture that supported the previous crop.
Recent studies have shown that continuous cropping of Spinach plants in infected soil enhances chances of infection recurrence because of the soil-borne pathogen.
A.occidentalis mainly causes infection to occur at the plant’s base, infecting leaves closer to the soil. To eliminate the disease, most growers make the common mistake of uprooting the whole plant and growing a new one in its place.
However, continuous cropping is not a viable solution to limiting the incidence of white spots on your Spinach plant. If anything, it makes the situation worse.
The fungal spores are microscopic and can easily be dispersed by wind/water to different regions of your garden/field.
Until the soil mixture is wholly replaced (and the inoculum removed) and proper plant hygiene is followed, preventing re-infection of white rust is not possible.
3. Overwatering
As mentioned earlier, fungal parasites love damp places. The tiny spores that serve as the fungus’s inoculum travel quickly through the water. Hence, overwatering your plant can cause serious problems.
Though the spinach plant requires a generous amount of water for the healthy growth of luscious green leaves, overwatering the soil promotes fungal attack.
A constantly soggy soil, moist leaves, and humid weather are crucial factors that will inhibit proper plant growth and will result in white spots that merge to form concentric rings.
Six Solutions to Treat/Prevent White Spots on Spinach Leaves
1. Use of Fungicides
One of the most effective methods to restrict the growth and spread of white rust is using fungicides.
Azoxystrobin and pyraclostrobin are famous fungicides used against A.occidentalis. They are synthetic products derived from a naturally occurring chemical compound, strobilurin.
These fungicides have been used to treat white rusts for many years now and have shown positive results. However, they are mainly used when the spread of disease is extensive.
In wide-spreading Spinach fields, the use of both these fungicides has ensured complete disease control.
2. Use of Copper Compounds
Liquid copper mixtures are available in the market, which gardeners can use to treat white rust. These mixtures contain compounds of copper hydroxide and copper sulfate.
These compounds are used along with metalaxyl to bring about the complete elimination of white spots on Spinach leaves.
You are required to spray the liquid on both the adaxial and abaxial surfaces of the leaves. Continue doing so for two weeks, and you will surely see some great results.
3. Better Irrigation Methods
The most common cause of white blister rust in Spinach plants is soggy soil and wet leaves. While planting a Spinach plant, you have to be careful in ensuring you do not overwater the soil.
An excellent method can be to use drip or furrow irrigation. In this way, the soil is sufficiently watered, and leaves remain soft and dry.
Ensure that you stay away from overhead irrigation methods or sprinklers as they will waterlog the soil and encourage pathogen attack.
4. Sulfur Sprays
Sulfur is anti-fungal and is hence effective in fighting against fungal diseases. Powdered sulfur or sprays are available in the market and should be used according to the manufacturer’s instructions on the packet.
In general, you should not use sulfur sprays if the climate in your particular city is above 80 degrees Fahrenheit (26 degrees Celsius). Moreover, while spraying/applying powdered sulfur on your Spinach leaves, make sure to cover your eyes and mouth.
Although sulfur is non-toxic, it can irritate the eyes and skin. Thus, precautionary measures are essential.
5. Crop Rotation and Tiling
If your Spinach plant has become entirely infected, dispose of it. But before you replant a new crop, you should get rid of the soil mixture and clean your whole garden.
Most likely, the fungus has spread far and wide within the soil, and replanting a new crop in the same soil will make it vulnerable to infection.
Another effective method is to till the garden soil. Tilling refers to the process of plowing the soil to mix the soil particles.
Deeper plowing ensures that fungi particles get diluted, and the hyphae are separated from their substratum (in this case, the roots). Hence, the severity of infection is reduced.
As stated earlier, it is strictly advised not to replant a crop of the same species in infected soil. Plant specialists recommend practicing crop rotation. Instead of growing cruciferous plants in an infected spinach bed, shift your plant to a new site.
6. Maintain Good Plant Hygiene
Aside from all these immediate treatments and preventions, it is also crucial to maintain proper plant hygiene at all times.
Cut out weeds, and make sure your soil is free of pests. Clean your leaves of dust, and ensure proper sanitation to prevent any spore formation.
Frequently Asked Questions about White Spots On Spinach
Can you eat Spinach with white spots?
The white spots on Spinach leaves indicate a fungal infection. Hence, the best advice is to throw them away. Eating such leaves will be detrimental to your health, and thus they should be strictly avoided.
How do I identify white rust on Spinach leaves?
Spinach plant infected with white rust typically has yellow leaves. The underside of the leaves has white pustules that are oval-shaped or appear as concentric rings. If your Spinach has chlorotic leaves with white spots, you can be sure the plant’s infected with white rust.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.