Rhubarb Rust Spots: Treating Brown Splotches On Rhubarb

Rhubarb Rust Spots: Treating Brown Splotches On Rhubarb

By: Amy Grant

Rhubarb is a cool weather, perennial vegetable that most people treat as a fruit, using it in sauces and pies. Rhubarb is easy to grow and, for the most part, pest- and disease-free. What causes rhubarb rust spots and what can be done for rhubarbs that have brown spots? Let’s learn more.

Rhubarb Spots on Leaves

There are a couple of diseases common to rhubarb, which may result in spots on rhubarb leaves. Usually leaf spots are more of an aesthetic issue and the unsightly spots don’t affect the edibility of the plant. The two most common diseases seen in rhubarb that result in spotted foliage are Ascochyta rei and Ramularia rei.

  • Ascochyta leaf spot is first seen as small, greenish yellow blotches (less than ½ inch (1.5 cm.) across) on the upper surface of the leaves. Gradually, the blotches develop white centers surrounded by a reddish border further bordered by a grayish-green area. After a few days, the infected areas turn brown, die, and fall out, creating a hole in the stalk which may be confused for insect damage. Ascochyta does not infect stalks but Ramularia does.
  • Ramularia leaf spot appears as small red dots (rhubarb rust spots) that enlarge to become round lesions of ½ inch (1.5 cm.) or greater in diameter. The spots become white, then tan with a purple border followed by stalk infection. Stalks develop a white fungus, gradually becoming brown as the tissue dies.

Both of these pathogens produce spores that spread to other plants via wind and splashing water, causing new infections 10-14 days later. The spores also remain in any debris left from season to season. Both Ascochyta and Ramulari fungi spread by infected rootstock.

Excellent sanitation in the garden is the key to thwarting both of these fungi. Select certified healthy rhubarb and plant in sunny, well-draining, fertile soil. Keep the area around the plants weed and debris free and remove and destroy any leaves that appear diseased. In severe cases of infection, a copper compound may be applied to control the leaf spot.

Another disease that may cause spotting is anthracnose stalk rot. Initially, the disease is seen as wilting leaves and large, lesions on the stems which enlarge rapidly and turn black. The stalks may become twisted and eventually collapse. As with the previous pathogens, good sanitation practices go a long way to controlling the disease. Remove and dispose of infected foliage or stalks. Also, fertilize the plant as soon as growth appears the next spring and then again as soon as the stalk harvest is over.

These diseases are most common in plants that are stressed, so improving their overall health is the key to decreasing the chances of infection.

What Else Causes Brown Splotches on Rhubarb?

While diseases may cause spots on rhubarb, cultural or environmental conditions may be responsible as well. Brown splotches on rhubarb may be the result of pesticide residue, salts, or a combination of both. These may begin as yellow patches seen upon the leaves, gradually becoming reddish brown.

Also, if your rhubarb has brown spots, the culprit may just be a healthy growing rhubarb. Yes, that is correct. Rhubarb needs to be divided every so often; 10 years is the maximum amount of time a rhubarb patch should go undivided. I’m not saying an undivided patch will die, just that a divided patch will flourish and thrive over an undivided one. It is possible that if you have rhubarb spots on leaves, all you need to do is dig them up and divide them.

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What are the spots on my rhubarb?

Ask Diarmuid + jobs for the week + plant of the week + the headling powers of grow-your-own

For the past few years in spite of changing the location of my rhubarb, the leaves continue to have brown spots something akin to measles, especially after the first picking. Any cure or ideas to prevent it?

It sounds like rhubarb leaf spot, which while unattractive is not actually killing your plant as you have discovered.

The disease is overwintering in the actual plant, not the soil, so changing locations won’t help. They appear after the first picking as the plant is under stress – heavy feeding will help here to give the plant a welcome boost.

Also keep the area free of weeds as these will be putting the plant under strain as it competes for nutrients and moisture. Remove any diseased leaves and destroy.

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Simple steps to treating leaf spot on rhubarb

Question: My rhubarb plants have spots all over their leaves. What are they and should I be concerned?

Answer: Rhubarb is a resilient plant, but it can develop two fungal leaf spots. Although the infections aren’t likely to kill the rhubarb, they can weaken it over time, which in turn could limit your crop.

Ascochyta leaf spot is the more common of the two in most regions. It affects only the leaves of the plant and looks different depending on the stage of infection.

In the beginning, yellow patches form on the foliage. Eventually the blemishes turn brown and then drop out of the leaf, leaving a small hole that could be confused with insect damage.

The other leaf spot is called ramularia. It attacks the leaves and edible stalks, so an extreme infestation could spoil your crop. Ramularia is occasionally called “red leaf” because the spots are crimson when they first appear.

As the leaf tissue dies, the spots turn tan, but they don’t fall out and leave a hole. After ramularia has spread to all the leaves, it will cause the stalks to turn brown and wither.

The best way to control leaf spot is to remove all infected leaves as soon as you notice the blemishes (do not throw them in a compost). The use of fungicides isn’t recommended not only is cutting back the contaminated plant parts enough to contain an outbreak, but harmful residues could end up on the edible stalks.

Rhubarb leaf spots are common in spring and early summer. A few sound horticultural practices should prevent an outbreak.

Buy healthy, disease-free plants from a reputable source, and plant them in nutrient-rich soil. Site rhubarb in a sunny spot, and don’t overwater, as moist, stagnant conditions are ideal for fungal growth.

It’s important that you remove dead foliage when the plant goes dormant the spores that cause leaf spots will survive in the withered matter throughout winter, lying in wait for new leaves to develop.

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Q. Is there a reason your rhubarb is standing in water?

Rhubarb prefers even moisture during the growing season (but does require well-drained soil), it most definitely does not like standing in water.

If it has been standing in water for a while, I would hazard a guess you're seeing a form of fungi (disease) brought on by crown rot.

Rhubarb isn't supposed to grow in the hot weather here in South Carolina. The soil is hard clay which takes forever to absorb water. Therefore I built a 2 inch high wall around it, which I fill with water everyday. The water takes about 5-10 minutes to be absorbed, which is why I can't just water it normally. Even with these measures it is sooooo hot here that if I miss a single day the plant wilts. Perhaps I should get some better soil.

Unless you create a better growing environment for your rhubarb I believe you will be forever fighting a losing battle.

Q. Can you grow them an area that gets some shade, preferably somewhere that gets morning & early afternoon full sun, then shade for the remainder of the day?

Or set something up to provide that kind of environment eg

Also rhubarb prefers soil with good organic content, which by its very definition means the soil will have good drainage.

Even in your climate rhubarb should only need approximately 2inches (50mm) of water a week, assuming you take care of the other requirements.

If you really want to grow rhubarb I think you might be best building yourself a raised bed:

  • You get to control the exact soil make-up, a major plus from what you've described about your native soil.


Rhubarb Growing Problems

by Linda Croudace
(Northumberland)

I am growing rhubarb and have had several pickings but today I found a lot of the stems covered in small clear, gelatinous blobs which has an unsightly effect under the skin.
Not sure if it is the snails/slugs for which my garden seems to be an ideal holiday destination OR just the really bad rain and humidity we have been having OR something else I haven't considered. Can anyone shed any light?

Comments for Rhubarb Growing Problems

Something is eating your rhubarb. That something could be caterpillars or beetles. Caterpillars usually eat the leaves so it's probably the Curculio beetle, sometimes called Snout beetle or Rhubarb weevil.

Rhubarb Curculios are usually red and grey/black with some yellow on their backs which rubs off. They are at least 1 cm long with curved snout-like mandibles which they eat with. When they attack rhubarb, not only do they eat it but they also make holes in rhubarb stems to lay their eggs in around springtime. The eggs drop to the surrounding ground and hatch in mid-late summer.

The clear jelly lumps on the rhubarb stalk are produced from the damaged holes. These holes are prone to decay, so controlling the beetles is important.

Try and keep dock plants away from your rhubarb as they can act as a host plant for curculios to breed on as well as a food source. To interrupt their breeding cycle you can clear away all debris surrounding the rhubarb plant, or cover the ground and any loose mulch with a flat layer such as newspaper, cardboard, matting etc.

Today the leaves of my rhubarb have brown patches, and when I pulled the stems and cut them for cooking I found the insides very red and a small - 1cm long - grey with dark head maggot crawling. One suggestion received was that this is the rhubarb weevil, but I can find nothing about this in the literature or online.


What to Do When You Find Holes in Rhubarb Leaves

By News Editor | August 8, 2016

Melinda’s Garden Moments is heard Mon.-Fri. at 7:45 and 10:45 a.m. and 4:45 p.m. on WHAV.

Rhubarb is a long-lived low maintenance vegetable with very few pests. So when holes appear in the leaves it may cause you to worry.

Several leaf spot diseases can cause small red or brown spots on the leaves and stems. The spotted tissue dies and often drops out, leaving holes in the leaves. Remove diseased leaves and stems as soon as they're discovered and do a thorough clean up in fall.

The rhubarb curculio is an occasional pest of rhubarb, feeding on the leaves and stems. You may notice sap oozing from the damaged stems or notches of missing tissue on the leaf edges. This pest is ½ inch long, has a large snout and its back is covered with a yellowish powder. Remove these pests when found and drop them in a can of soapy water.

Keep the garden weed-free and avoid overhead watering to keep plants healthy.

A bit more information: Some weeds are hosts for these diseases and insects that attack rhubarb. Managing weeds can reduce the risk of pest problems. Fortunately, established plants survive and recover with proper garden cleanup and care.


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