Plant diseases are infuriating! – It is just horrifying to see all our hard work withering on the vine. Preventative medicine as opposed to treating symptoms after they have become a problem is always the best approach so here are a few tips to keep your garden healthy and disease free:
When shopping for plants look for disease resistant varieties and choose plants that are vigorous and healthy. Shopping with a reputable grower is always a wise choice.
If you notice any sign of disease on plants immediately remove damaged parts and if necessary the entire plant. Composting will not always kill pathogens which can spread to additional plants or even overwinter in the soil so diseased plants should be sent out with the trash or burned.
Choose the right plant for the right location – If a plant is stressed because it is in the wrong spot – has too much sun or too much shade it will be more susceptible to disease. The same it true of water and fertilizer – the wrong amount of either one can cause more damage than good.
Rotation of vegetable crops will help break the cycle of disease by eliminating the build up of diseased organisms in one spot.
Over crowding of crops can create too much humidity and fungal disease. Keep air flow moving by good planning and thinning.
Keep the garden tidy by turning under finished crops before disease causing organisms can take over. And clean tools and especially pruners in between uses to keep from spreading disease from an infected host.
Preventative treatments like dormant sprays can be a safe way to protect plants – be sure to follow instructions carefully.
Because of our high rainfall and mild climate fungal and bacterial diseases are the most common problems in Portland. Here are a few to keep an eye out for:
AIRBORNE FUNGAL DISEASES
This fungal disease looks like a grey powdery mold. It primarily appears on vegetable, bulbs and some perennials. Strawberries and zucchini are especially susceptible. Look for wet looking spots on leaves or fruit which will turn into a powdery grey mass. The entire plant can sicken and die so deal with it as soon as you spot it.
Air circulation is the best defense as this fungus prefers warm and humid areas. More sun also helps so pruning neighboring plantings can be useful so avoid wetting the leaves of susceptible plants while watering. Finally – remove and destroy any infected plant parts and wash tools and hands after.
Another fungal disease that commonly affects PNW gardens is powdery mildew. Late Summer is the time to look for white powder splotches that envelop leaves and stems. Cucumber, beans, zucchini, squash and roses are among it’s favorite victims. Although it can cause serious damage if left unchecked it is possible to minimize spread and still reap a harvest from infected crops.
Making space for air flow and maximum sun is essential. Well watered plants are less likely to fall victim but be sure to water from below and avoid water on leaves. Remove and destroy damaged foliage. A solution of 1 T baking soda to 1 gallon water and a tad of liquid soap spayed onto the leaves will also help.
There are a gob of different fungi that produce rust but the prevention, identification and treatment is similar. Orange to yellow spots present in speckled formation on the leaves. Spring or late Summer is the usual time for it to appear as the fungi prefer mild days and cooler damp nights.
Again overhead watering should be avoided to restrict the spread of rust. More sun also helps. Remove rusted leaves and garden debris to prevent the cycle of reinfection.
Commonly attacks apples and pears in the Spring right after flowering. It looks just like it sounds with with yellow to brown to black blotches on the leaves and sometimes the trunk. Leaves can eventually drop and in extreme cases defoliation may occur. Fruit can also appear scabbed but is still edible.
Remove and destroy damages areas. Pruning for air circulation is helpful but in extreme cases a copper spray can at lease prevent against another infection the following year.
SOIL/WATERBORNE FUNGAL DISEASES
Ramorum blight and tip dieback
The symptoms vary widely according to the host plant but generally look for bark cankers especially ones that are bleeding or girdle the entire circumference of the trunk. Leaf spotting or blotches appear in areas of the leaf that can collect water such as at the tips or the midribs.
This disease only first appeared in 1995 but has quickly spread. It can affect many types of native oaks, rhododendrons, myrtle, bay laurel, madrone, cascara, redwood, douglas fir, kinnikinnick, salmonberry, and evergreen huckleberry. It also attacks viburnum, camellias and pieris among others.
The risk of root rot comes in the winter as water drenched soil robs roots of oxygen and makes them susceptible to this soil fungus. Signs may not appear on the plant until the early summer when the plant is unable to draw water through its damaged roots. Plants may appear wilted with leaves eventually browning but adding water will not help. Prevention is the best approach start with healthy plants and create good drainage before planting. A mycelium application will improve soil conditions and root health. Mulching can also help to discourage spread by cooling the soil. If unable to recover replace plants with a resistant variety that is suitable to the location.
Look for one single branch showing signs of stress – wilting or the leaves turn brown while the rest of the plant looks perfectly fine. Often a long canker will also appear in the affected area and a ring will be present in the cross section of a cut branch. This soil borne fungus will attack many plants but maples are especially at risk. Complete lists are available online.
A similar disease attacks tomatoes, peppers and eggplants.
Established trees can sometimes be saved by removing affected branches and building the up soil health. Apply a thin compost but avoid high nitrogen, potassium is better. Clean tools immediately after use as the disease can be easily spread to other plants. If the plant needs to be removed replace with a resilient species.
Rapidly wilting leaves and dark splotches that manifest in the rainy spring can mean a bacterial infection. Remove and destroy affected parts and plants. Wash tools between each cut to avoid spread and wash hands before moving to another area of the garden.
This bacterium can move quickly and so the symptoms progress rapidly. Leaves and entire branches can redden and drop within days. Affected areas should be removed and burned as soon as detected. Cuts should be made at least 12 inches below the affected area. Tools need to be cleaned between cuts. This bacterium is transmitted by pollinating insects and infected plants can act as hosts and spread rapidly to other plants.
The Pacific Northwest Plant Disease management handbook is available online and has an exhaustive list of diseases. It is organized by host so it makes identifying the disease a little easier. Be sure you have the right diagnosis before beginning a treatment and please try natural methods before resorting to harsh chemical applications.