Why Do Bean Plants Develop White Spots on Their Leaves, And Is It a Problem?

leaf bean with red spider mite damage

During the summer season, bean plants may become unhealthy looking, with a mottled or stippled pattern of tiny whitish speckled spots, short streaks or short lines on the upper surface their leaves.

These affected leaved may eventually turn yellow, then brown, and become dry and shriveled.

This is damage caused by a tiny sap-sucking pest, the two-spotted spider mite, also known as the red spider mite (Tetranychus urticae), which is barely visible to the naked eye, being only 0.3-0.5mm long, that live on the underside of leaves.

Larger female and smaller male two-spotted mites (Source: University of Warwick, image modified and enhanced by Deep Green Permaculture)
Larger female (0.5mm long) and smaller male (0.3mm long) two-spotted spider mites (Source: University of Warwick, image modified and enhanced by Deep Green Permaculture)

Feeding damage caused by spider mites is first observed on the lower part of a plant and gradually moves upward as the mite population increases in numbers.

Spider mites will usually feed exclusively on one plant until it starts to weaken and decline, and then they will move on to a new plant.

A common characteristic of spider mite infestation is the presence of fine silk webbing similar to spider webs that may appear on new growth and between leaves but may cover whole leaves. These webs are used to line feeding areas to protect the mites, and to help them to be dispersed by the wind to move to new plants.

Heavy infestations of two‐spotted spider mites can cause severe crop damage and seriously weaken plants, causing defoliation and reductions in crop quality and yields.

Spider mites prefer a dry environment, so rainy weather helps to keep population levels in check.

How to Control Red Spider Mites

wettable sulphur and horticultural oil mite controls
Wettable sulphur and horticultural oil, as well as hortucultural soap, are effective and safe organic controls of red spider mites

There are many safe and environmentally friendly ways to prevent and control spider mites, for more detailed information see the article – The Best Ways to Control Two-Spotted Spider Mites

To offer a quick run-down on the most popular methods, here’s a short list:

  • Hosing – some gardeners simply hose the pests off the plants, which reduces their populations significantly.
  • Horticultural oils and insecticidal soaps – are organically approved non-toxic contact sprays that smother the pests so they can’t breathe, and only work when sprayed directly on a pest to cover its body.
  • Wettable sulphur – is an organically approved miticide used to eradicate pest mites and is also used as a fungicide. This colloidal form of elemental sulphur is a fine yellow-brown powder which dissolves in water and is designed to be sprayed. Regular agricultural sulphur which is used as a soil amendment, is not the same, as it can’t be dissolved in water and can’t be sprayed. Use the right one!


One thought on “Why Do Bean Plants Develop White Spots on Their Leaves, And Is It a Problem?

  1. What Fruit & vegetables can you grow that are strong & fairly immune to insect attack???

    For instance this year I have given up on planting Beans because of insect attack & weak crop yield.

    And I have especially given up on tomatoes as the green shield vegetable beetle destroyed virtually my entire crop of tomatoes last year.

    So what fruit & vegetables can the home gardener just plant & leave, without having to worry about insect & fungal attacks??

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