The Inspection of Imported Plants

Donald P. Limber, Paul R. Frink.

Federal plant quarantines have been in effect in the United States since 1912. They do not prevent the importations of large numbers of plants each year. The plants offered for introduction must be examined to determine that they are free from plant pests that are not present in our country or are not widely established here. This first examination at the ports of entry searches out all plant pests insects, fungi, virus diseases, and nematodes.

The Bureau of Entomology and Plant Quarantine is responsible for the pest-risk problem in the importation of plants. Two States, California and Florida, collaborate with the Federal Government at the ports within their borders. All States cooperate in the follow-up inspections that are given certain genera of plants when they are grown in the field under postentry quarantine.

The inspector's basic tools are a hand lens and a microscope. He examines the imported plant material to see if any plant diseases are present and identify the ones he finds. On the identification depends the decision as to whether those plants should be rejected, treated, or released. If he cannot identify the disease, the inspector holds the shipment and refers a specimen to the Washington office for determination by specialists.

The very nature of plant diseases makes it hard to enforce plant quarantines. Bacteria and the spores of the higher fungi that spread the diseases are so minute that they are usually invisible without magnification unless massed in large numbers. Even when the spores have germinated and invaded the tissues of the plant, evidence of the developing disease ordinarily does not appear at once. Leaves inoculated with Colletotrichum cypripedii show the first symptoms in 15 or 16 days. That is about the normal period for the incubation of many other diseases, but some may appear in as few as 4 days and others only after a month or longer.

Few fungus diseases can be eradicated when they are present in living plants. Therefore the plants on which a new disease is found usually are rejected. There are some exceptions. Hot-water treatments for nematode diseases and a few fungi (such as the leaf smut of rice, Entyloma oryzae, and the mint rust, Puccinia menthae) under favorable conditions may eliminate the parasites. Another example: The citrus seeds may carry the bacteria Xanthomonas citri, which cause the citrus canker. Citrus seeds immersed in a solution of 1 part peroxide of hydrogen to 2 parts of water for 10 minutes are completely cleansed of viable citrus canker bacteria.

THE PLANT QUARANTINE INSPECTOR at Houston, Tex., made his customary examination of the stores of a cargo vessel from Japan. The fruits and vegetables in ship's stores, though for use on the ship, may be a serious risk. The crew members may attempt to smuggle fruit ashore. Peelings and spoiled stores may be thrown into harbors and washed ashore.

In the stores the inspector found five citrus fruits on whose rinds were numerous small, round, corky spots. The fruits were confiscated and destroyed, as citrus from Japan is prohibited entry into the United States.

Samples of the rind bearing the spots were sent to a Bureau mycologist at Hoboken, N. J. All five fruits were infected with citrus canker, Xanthomonas citri, a highly destructive disease of citrus and one that is believed to have been completely eradicated from the United States after long and expensive effort.

Citrus canker was found in passengers' baggage and ships' stores on 10 occasions in a single month at the port of San Francisco an illustration of the need for continuous vigilance.

AT THE OLD inspection house in Washington, D. C., which stood on the corner of Constitution Avenue at Twelfth Street, an inspector was examining some orchids from the wilds of Brazil. Some small patches of dusty, yellow material on a leaf caught his eye.

It seemed harmless at first glance and resembled amorphous materials sometimes found on orchid leaves. Upon turning the leaf over, however, he saw that there was a yellowing of the tissue extending through the leaf. Other leaves with more numerous spots were found. Some of the leaves were dead.

When sections were made, the dusty material proved to be the uredospores of the rust, Hemileia oncidii. All the infected leaves were removed and destroyed. The plants were then disinfected by a dip in bordeaux mixture. Hemileia oncidii and other rusts of orchids, which have also been intercepted many times, are not known to be established in the United States.

A LARGE SHIPMENT of lily-of-the-valley pips (Convallaria majalis) arrived on the docks at New York in 1950. The pips originated near Hamburg, Germany. The inspectors at the port gave them the usual thorough examination on the pier, as these plants are a known host of the stem and bulb nematode (Ditylenchus dipsaci).

No Ditylenchus was found, but the roots carried some sandy soil. Soil is a serious hazard in quarantine enforcement. It may carry insects, particularly the larvae and pupal stages, various soil-borne fungi, and plantnematodes. Imported plant material therefore must come with clean roots. The pips were ordered to be sent to the Hoboken Inspection House for cleaning. At Hoboken the washings from the roots were examined carefully for plant pests, and finally processed to recover any nematode cysts that might be present.

The plant pathologist, looking down through the microscope at the debris floating in the dish beneath the lens, observed a smooth, dark-red, spherical body with a short neck, floating around with the debris. The whole object was less than I millimeter in diameter. Here was a truly dangerous immigrant the cyst of the golden nematode (Heterodera rostochiensis) filled with living eggs.

Further search revealed that it was not a lone cyst, but that the whole shipment of pips was heavily contaminated with golden nematode cysts, by hundreds at least, and possibly by thousands. Each cyst contained 10 to 400 eggs. It is quite probable that this one shipment of lily-of-the-valley, after being distributed by sale to florists for growing, might have established the nematode in not one but many new areas. Its introduction into our remaining potato-growing areas would be a disaster as it has proved to be on Long Island and in some parts of Europe.

The interception of the golden nematode on these Convallaria was not the first interception of this pest nor by any means the last. The cysts are found frequently not only in soil with plants, but they have also been intercepted adhering to straw and burlap bagging. The menace they present is heightened by the longevity of the eggs within the cysts. H. rostochiensis eggs have been known to hatch after 8 years.

Part of the Convallaria shipment was treated with hot water. The pips were immersed in hot water at 118 F. for 30 minutes, then removed and cooled with water after draining for 5 minutes. The treatment must be given with care as the margin between a complete kill of the nematodes and serious injury to the plants is very small. The plants not treated were forced in postentry quarantine in isolated greenhouses. After the flowers were harvested the plants were destroyed, and the soil and the benches then sterilized with steam.

Our three examples illustrate the varied nature of the inspector's problem. The virus diseases are especially difficult to detect in dormant plants, the condition in which most nursery stock is imported. It was largely to overcome this difficulty that postentry quarantine was devised.

POST-ENTRY QUARANTINE is the requirement that some genera of plants be grown under observation of State and Federal inspectors, usually for two growing seasons, before they are released for sale or distribution. The plants subjected to this treatment are the ones known to be the hosts of some serious plant disease that does not occur here or is restricted in distribution in the United States. In most instances the plants are prohibited from the countries in which the disease is known to occur. The post-entry provision- then applies only to other countries in which it might be present but unreported.

Rose wilt virus (Marmor flaccumfaciens) occurs in Italy, Australia, and New Zealand. Rose plants or cuttings are prohibited from those countries. If a nurseryman wishes to import the material from any other foreign country he will be required to make a legal agreement that he will grow the material at a designated place, accessible to the State nursery inspector and the Federal plant quarantine inspector until it is released. In each of the following two growing seasons the State nursery inspector will inspect the plants several times. Sometimes he will be accompanied by an inspector from the postentry section of the division of plant quarantines. The plants will be examined particularly for any evidence of rose wilt virus, but also for other foreign diseases or insects which may have accompanied them.

Postentry inspectors also must gather from the published records information on the foreign plant diseases which we are attempting to exclude. Summaries of the information are then distributed to the State inspection services. In that way the State inspectors can know the proper time to inspect roses, hops, or other postentry plants in their areas, and something of the appearance of the foreign diseases.

Daphne mosaic virus was found in 1950 in a lot of 250 Daphne mezereum plants from Holland. Then the disease was known only in Australia and New Zealand. It was the basis for the inclusion of daphne in the post-entry list. The plants were promptly destroyed under the supervision of the State nursery inspector.

At the end of the second growing season those postentry plants which have remained free of diseases and insects new to the United States are released from quarantine.

The postentry inspection makes it possible to examine the plants when they are in leaf and at times favorable for detection of the particular diseases for which they are quarantined. In 1953 there were 50 genera of plants that must be grown in postentry quarantine when imported from certain parts of the world. Besides them, a blanket provision includes fruit and nut plants.

DONALD P. LIMBER has been employed as a plant pathologist in the Department of Agriculture's plant quarantine work since 1924. He is now plant pathologist of the postentry section of the Bureau of Entomology and Plant Quarantine stationed at Hoboken, N. J.

PAUL R. FRINK, a native of Nebraska, received his graduate training in plant pathology at the University of Nebraska. He has worked with several Government agencies since 1931 and with the division of plant quarantines since 1942. He is employed as a plant quarantine inspector at the San Francisco Inspection Station.