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Rust Spots on Leaves during Flowering — Causes & Treatment

Rust Spots on Leaves during Flowering — Causes & Treatment

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Rust spots are caused by a fungus and it is a huge problem for gardeners of all types. Indoors, under grow lights, or in a raised garden bed. It can infect thousands of host plants. 

Just when you get your plants blooming, along comes a fungus to ruin your entire season. 

On flowering plants, blooms fade fast, flowers drop, leaves wilt, and you wind up with a freckled plant with multiple rust spots.  

Read on to discover how it starts and spreads, and how to prevent its horrible fungal spores from germinating so you can stop the spread and keep your plants in bloom. 

Rust is a fungal disease that attacks plants more during flowering. While the plant pours energy into blooming, its leaves are weakened. Weakened foliage is a playground for fungus to thrive. Once it starts, it spreads fast. Elemental sulfur is the solution to stop the spores from germinating. 


What causes rust fungus 

Knowing how diseases start is the first step in understanding how to control, limit, and stop the spread. 

The conditions that all rust fungus’ need for germination are warm temperatures, high humidity, and moisture. 

In the case of rust infections, the temperatures that encourage spore germination are between 60-Fahrenheit and 80-Fahrenheit. 

It is more common in a closed environment, such as in a greenhouse environment or indoors under grow lights, but since it is an airborne disease, it can affect any plant, indoors or outdoors in the garden. 

On houseplants and garden plants, the flowering stage is when plants are most susceptible. The plants are using most of their energy for blooming leaving the leaves susceptible to infection. 

It should be noted that rust spots can infect plants at any stage. 

It starts on the underside of leaves, then spreads. By the flowering stage is when it spreads faster. Chances are, the spores have been there all along, but with the leaves in a weakened state, the spores become more visible. 


The life cycle of rust spot disease

The reason why rust is so prevalent is that it needs at least two host plants to complete its life cycle. Thousands of plants are host plants. 

Early infections start on the underside of leaves as white spots that are easily mistaken for powdery mildew. However, as it progresses, the fungal spores change color. 

Rust fungus’ go through five stages in their life cycle, each time changing color. 

It starts as a white fungus, then turns yellow, then orange, then brown, then black, at which point the leaf dies and drops from the plant. 


Controlling the spread of rust spot disease 

In the center of rust spots is a small pustule that is darker in color. That will eventually rupture, at which point the fungal spores become airborne. That is how it spreads.

The closer your plants’ leaves are together, the faster the infection travels. 

Whenever you see fungal rust spots on any leaf…


Stop applying fertilizer 

When fighting any disease, or even an insect infestation, you do not want the plant to focus on new growth until you have the problem under control. 

The reason to stop applying fertilizer is that new growth is incredibly weak. The leaves have the nutrients they need but lack the structural strength to fight off diseases. 

Rust pathogens are obligate parasites so they can only survive on living plant tissue. 

The healthier the leaves, the faster the fungus spreads. Weak new growth will have all the nutrients the fungus needs to thrive, leaving less for the plant to flourish. 

The cumulative effect will be a plant that focuses on survival instead of flowering. 

Later in the season when plants start flowering, adding fertilizers for flowers to bloom more that’s rich in either nitrogen or phosphorous will encourage new foliage growth and promote flowering. 

All those rich nutrients will encourage more rust spots to develop. That is why rust damage is more visible during flowering. It is just because there is a lot of nutrients available to it. 

Cut back on the fertilizer. Focus on treating the leaves with a fungicide. 

On houseplants or plants growing in containers, you can go further to stop the spores from germinating. Move the container out of sunlight and/or reduce watering to slow new growth down. 


Use mulch to slow the spread of rust fungus 

On outdoor plants, and in raised garden beds, mulch is the simplest way to slow the spread of rust fungal spores. It will not stop it from spreading because the fungus is dispersed by wind and water. 

The pustules within the rust spots on leaves burst and it is when it bursts that the fungus drops to the ground or spreads by the wind. The bulk of the spores fall to the ground though. 

By having a thick layer of disposable mulch, such as straw or other garden debris, you can simply remove the mulch periodically and trash it. If you have an incinerator, burn infected mulch. 

Do not compost diseased mulch material as rust pathogens can survive in compost and overwinter in the soil. 


Treat rust spores with elemental sulfur

Elemental sulfur has a dual benefit on plants. Sulfur is the fourth important nutrient for plants after NPK essential nutrients

The added benefit of elemental sulfur is that it works as a preventative fungicide. 

It will not kill rust fungus, but it will prevent germination, thus stopping the spread. For it to work though, repeat applications are needed.  

For larger outdoor plants like trees and shrubs that get infected with a rust spot disease, applying a dusting of elemental sulfur powder, or a foliar spray on the upper and lower side of all the leaves will prevent overwintered fungal spores from germinating. 

On larger bushes, shrubs, and trees in the garden, applying a dusting or spray of elemental sulfur from early spring each season stops the disease from returning.


Frequently Asked Questions related to rust spots on leaves during flowering


How are rust spots different from brown spots? 

Rust spot is a fungal disease that wipes off the plant foliage similar to powdery mildew. Brown spots are either water mold on the leaf surface or an infection inside the leaf, not on the surface. Wipe your finger over the leaf and if the brown wipes off, it is a rust fungus.  


Will rust spots kill the plant? 

If nothing is done to prevent spores from germinating, rust will continue to attack the plants’ foliage. The more spores germinate, the less photosynthetic surface the plant will have. It stops photosynthesis and hinders respiration. The more rust spots there are, the more likely the plant will die.