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How to Monitor Rate of Spread of Phylloxera in Your Vineyard

Additional Fact Sheets on Phylloxera:

Contributing List of Authors: Ed Hellman. Oregon State University. This fact sheet and others on grape phylloxera were produced as a set by a phylloxera task force at Oregon State University: Bernadine Strik, Extension Berry Crops Specialist; M. Carmo Candolfi-Vasconcelos, Extension Viticulture Specialist; Glenn Fisher, Extension Entomologist; Edward Hellman, Extension Horticulture Agent; Steven Price, Post-doctoral Research Associate, viticulture; Anne Connelly, Master’s student, horticulture; and Paula Stonerod, Research Aide, horticulture. The authors of this fact sheet acknowledge the help and guidance of others on this task force.

It may take several years from an initial infestation before you can actually find phylloxera on vine roots (see Sampling Vines to Confirm Presence of Phylloxera). This aphid-like insect is very small and spends most of its life cycle below ground. Above-ground forms have been found in the Pacific Northwest, but are difficult to detect. Susceptible vines that are infested show typical symptoms of decline due to root feeding and decay (see Phylloxera: What is it?).

Once you know your vineyard has an infestation, you should monitor rate of spread and decline in production to estimate how long the vineyard will remain productive. You should also consider how (or if) to make the transition to a resistant or grafted vineyard (see Replanting Options for Establishing Phylloxera-resistant Vineyards).

Monitoring rate of spread within your vineyard is based on recording the rate and direction of spread of above-ground symptoms, not the phylloxera, because infestations on roots can be difficult to see (see Sampling Vines to Confirm the Presence of Phylloxera). Keep in mind that once an infested site is confirmed, the phylloxera will have likely spread in a much wider area than is evident from the above-ground symptoms. This is because it takes a while for populations to build to a level where the vine is stressed and vigor is reduced. It’s important to realize that when you’re monitoring rate of spread through above-ground symptoms (the only practical way) you are not monitoring the actual movement of the pest. The insect may already be in the entire block or vineyard, as symptoms may show first in “naturally” weaker areas of the vineyard (see Phylloxera: What is it?).

The following methods can be used as an estimate of rate of spread of above-ground symptoms:

Visual counting of affected vines

Infestations often first appear as a lens-shaped area of weak vines (see Sampling Vines to Confirm the Presence of Phylloxera). However, if an infestation originated from infested plant material, this may not be the case. Phylloxera-infested vines that are succumbing to high feeding pressure will show reduced vigor and lighter green-colored foliage. The easiest, but least accurate, way to estimate rate of spread is to count the number of vines assumed to be infested within each lens. Do this in the fall just before or just after harvest when symptoms are most apparent. Infested vines often drop their leaves earlier than healthy vines or the foliage yellows more quickly. However, other factors, such as other pests, nutritional imbalances, or drought, can cause similar symptoms.

Counting the number of affected vines annually will give you a rough estimate of the economic rate of spread (rate of increase of non- or low-producing area). Subtract the number of vines affected last year from the number affected in the current year and divide by the number affected last year. This will give you the percent increase in size. For example, if 50 vines show reduced vigor this year and 20 did last year, the rate of spread would be: (50-20)/20 = 1.5 or a 1.5X (150%) rate of spread. Doing this for a few years will give you an idea of how quickly the vineyard will succumb to the infestation. In Oregon, we’ve seen rate of spread vary from 1.5X (from a point source infestation in an older vineyard) to 10X (in a seven-year-old vineyard where phylloxera were introduced on the plant material).

Keep in mind that you need to look out for “satellite infestations”, or new areas of infestation that occur separate from the initial finding. This is typical in infested vineyards because weaker vines succumb to infestations faster than more vigorous ones.

Vigor ratings

This is a modification of the counting system described above and is more accurate if the same person rates vine vigor each year. Document the size of the declining area(s) in your vineyard by counting the number of dead vines and ascribing a vigor rating to the affected vines in the area (for example, 1 = healthy; 2 = mildly stunted or reduced growth; 3 = severely stunted; 4 = dead). Map the area on graph paper to keep accurate records.

A modification of this system is to keep records of pruning weights in vineyard blocks or affected areas. As vine vigor declines due to infestation, pruning weight should, also. Using one or two transects through an infested area to monitor pruning weight and yield per vine will tell you a lot about rate of spread of phylloxera and its economic impact.

Aerial photography

This is the most accurate method for evaluating vine decline in the vineyard. It is also the only method that can effectively be used to monitor rate of spread within a region.

Infrared aerial photography is superior for detecting changes in vine vigor. Healthy vines show up as bright or dark red whereas weak and declining areas show up as lighter shades of red to no red. Keep in mind that healthy weeds or cover crops also show as a dark red color. Weak areas due to shallow soils will usually not enlarge in size annually unless erosion is occurring. Weak areas are easily spotted from the air. These weak areas can be inspected with follow-up ground surveys to determine the possible reasons for vine decline. Obviously, the photographic resolution and altitude at which the vineyard is photographed will determine the minimum size of weak areas to be detected (i.e. one or 10 vines).

Not all weak areas in vineyards are due to phylloxera infestation. Aerial photography has effectively monitored vineyard changes. Many growers find that photographs taken every two to three years have been adequate in detecting vineyard problems and phylloxera spread.